From 36c788701f98c60fbea417198dbbb3407fa599bc Mon Sep 17 00:00:00 2001
From: Vladimir Sitnikov <sitnikov@netcracker.com>
Date: Thu, 6 Jun 2024 20:26:14 +0300
Subject: [PATCH] WIP: fixup nullability annotations and generic variance

---
 .../main/java/net/jqwik/api/Arbitraries.java  | 78 +++++++++----------
 .../main/java/net/jqwik/api/Arbitrary.java    |  4 +-
 api/src/main/java/net/jqwik/api/Builders.java |  8 +-
 .../main/java/net/jqwik/api/Combinators.java  | 78 +++++++++----------
 .../java/net/jqwik/api/RandomGenerator.java   |  3 +-
 .../main/java/net/jqwik/api/Shrinkable.java   |  2 +-
 api/src/main/java/net/jqwik/api/Tuple.java    | 34 ++++----
 .../jqwik/api/arbitraries/ArrayArbitrary.java |  6 +-
 .../api/arbitraries/FunctionArbitrary.java    |  4 +-
 .../api/arbitraries/IteratorArbitrary.java    |  6 +-
 .../jqwik/api/arbitraries/ListArbitrary.java  |  6 +-
 .../jqwik/api/arbitraries/MapArbitrary.java   |  4 +-
 .../jqwik/api/arbitraries/SetArbitrary.java   |  6 +-
 .../api/arbitraries/SizableArbitrary.java     |  4 +-
 .../api/arbitraries/StreamArbitrary.java      |  6 +-
 .../api/arbitraries/StreamableArbitrary.java  |  4 +-
 .../api/arbitraries/TraverseArbitrary.java    |  4 +-
 .../jqwik/api/arbitraries/TypeArbitrary.java  |  4 +-
 .../api/statistics/StatisticsCoverage.java    |  2 +-
 .../customized-parameter-generation.md        | 10 +--
 .../StatisticsCoverageExamples.java           |  5 +-
 .../engine/facades/ArbitrariesFacadeImpl.java | 28 +++----
 .../engine/facades/CombinatorsFacadeImpl.java |  2 +-
 .../statistics/StatisticsCollectorImpl.java   |  6 +-
 .../arbitraries/DefaultArrayArbitrary.java    |  2 +-
 .../arbitraries/DefaultFunctionArbitrary.java | 14 ++--
 .../arbitraries/DefaultIteratorArbitrary.java |  2 +-
 .../arbitraries/DefaultListArbitrary.java     |  8 +-
 .../arbitraries/DefaultSetArbitrary.java      |  2 +-
 .../arbitraries/DefaultStreamArbitrary.java   |  2 +-
 .../arbitraries/EdgeCasesSupport.java         |  2 +-
 .../arbitraries/FrequencyOfArbitrary.java     |  4 +-
 .../FromGeneratorWithSizeArbitrary.java       |  4 +-
 .../properties/arbitraries/LazyArbitrary.java |  4 +-
 .../arbitraries/LazyOfArbitrary.java          |  6 +-
 .../arbitraries/OneOfArbitrary.java           |  5 +-
 .../arbitraries/RecursiveArbitrary.java       |  6 +-
 .../combinations/DefaultListCombinator.java   |  2 +-
 .../exhaustive/ExhaustiveGenerators.java      |  2 +-
 .../FlatMappedExhaustiveGenerator.java        |  6 +-
 .../randomized/AbstractFunctionGenerator.java |  6 +-
 .../randomized/ConstantFunctionGenerator.java |  2 +-
 .../randomized/FrequencyGenerator.java        |  2 +-
 .../randomized/FunctionGenerator.java         |  2 +-
 .../randomized/RandomGenerators.java          |  4 +-
 .../shrinking/ChooseValueShrinkable.java      |  4 +-
 .../support/ChooseRandomlyByFrequency.java    |  6 +-
 .../statistics/StatisticsCoverageTests.java   |  8 +-
 kotlin/build.gradle                           |  1 +
 .../jqwik/kotlin/api/BuildersExtensions.kt    |  6 +-
 .../net/jqwik/kotlin/api/SequenceArbitrary.kt |  4 +-
 .../kotlin/api/SizableArbitraryExtensions.kt  |  2 +-
 .../jqwik/kotlin/CollectionExtensionsTests.kt |  4 +-
 .../net/jqwik/kotlin/InternalModifierTests.kt |  6 +-
 54 files changed, 231 insertions(+), 211 deletions(-)

diff --git a/api/src/main/java/net/jqwik/api/Arbitraries.java b/api/src/main/java/net/jqwik/api/Arbitraries.java
index b46c0943c..cefb5d81a 100644
--- a/api/src/main/java/net/jqwik/api/Arbitraries.java
+++ b/api/src/main/java/net/jqwik/api/Arbitraries.java
@@ -25,13 +25,13 @@ public static abstract class ArbitrariesFacade {
 			implementation = FacadeLoader.load(ArbitrariesFacade.class);
 		}
 
-		public abstract <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> all);
+		public abstract <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> all);
 
 		public abstract <M> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> actionArbitrary);
 
-		public abstract <T> Arbitrary<T> frequencyOf(List<Tuple2<Integer, Arbitrary<T>>> frequencies);
+		public abstract <T extends @Nullable Object> Arbitrary<T> frequencyOf(List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies);
 
-		public abstract <@Nullable T> Arbitrary<T> just(@Nullable T value);
+		public abstract <T extends @Nullable Object> Arbitrary<T> just(T value);
 
 		public abstract IntegerArbitrary integers();
 
@@ -57,36 +57,36 @@ public static abstract class ArbitrariesFacade {
 
 		public abstract <T> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<TypeUsage, Arbitrary<Object>> noDefaultResolver);
 
-		public abstract <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier);
+		public abstract <T extends @Nullable Object> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier);
 
 		public abstract <T> TypeArbitrary<T> forType(Class<T> targetType);
 
-		public abstract <K, V> MapArbitrary<K, V> maps(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
+		public abstract <K extends @Nullable Object, V extends @Nullable Object> MapArbitrary<K, V> maps(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
 
-		public abstract <K, V> Arbitrary<Map.Entry<K, V>> entries(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
+		public abstract <K extends @Nullable Object, V extends @Nullable Object> Arbitrary<Map.Entry<K, V>> entries(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
 
-		public abstract <T> Arbitrary<T> recursive(
-			Supplier<Arbitrary<T>> base,
-			Function<Arbitrary<T>, Arbitrary<T>> recur,
+		public abstract <T extends @Nullable Object> Arbitrary<T> recursive(
+			Supplier<? extends Arbitrary<T>> base,
+			Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 			int minDepth,
 			int maxDepth
 		);
 
-		public abstract <T> Arbitrary<T> lazyOf(List<Supplier<Arbitrary<T>>> suppliers);
+		public abstract <T extends @Nullable Object> Arbitrary<T> lazyOf(List<? extends Supplier<? extends Arbitrary<T>>> suppliers);
 
 		public abstract <T> TraverseArbitrary<T> traverse(Class<T> targetType, Traverser traverser);
 
 		public abstract Arbitrary<Character> of(char[] chars);
 
-		public abstract <T> Arbitrary<T> of(Collection<T> values);
+		public abstract <T extends @Nullable Object> Arbitrary<T> of(Collection<T> values);
 
-		public abstract <T> Arbitrary<T> create(Supplier<T> supplier);
+		public abstract <T extends @Nullable Object> Arbitrary<T> create(Supplier<T> supplier);
 
-		public abstract <T> Arbitrary<List<T>> shuffle(List<T> values);
+		public abstract <T extends @Nullable Object> Arbitrary<List<T>> shuffle(List<T> values);
 
-		public abstract <T> Arbitrary<T> fromGenerator(IntFunction<RandomGenerator<T>> generatorSupplier);
+		public abstract <T extends @Nullable Object> Arbitrary<T> fromGenerator(IntFunction<? extends RandomGenerator<T>> generatorSupplier);
 
-		public abstract <T> Arbitrary<T> frequency(List<Tuple2<Integer, T>> frequencies);
+		public abstract <T extends @Nullable Object> Arbitrary<T> frequency(List<? extends Tuple2<Integer, T>> frequencies);
 	}
 
 	private Arbitraries() {
@@ -99,7 +99,7 @@ private Arbitraries() {
 	 * @param <T>       The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <T> Arbitrary<T> fromGenerator(RandomGenerator<T> generator) {
+	public static <T extends @Nullable Object> Arbitrary<T> fromGenerator(RandomGenerator<T> generator) {
 		return fromGeneratorWithSize(ignore -> generator);
 	}
 
@@ -111,7 +111,7 @@ public static <T> Arbitrary<T> fromGenerator(RandomGenerator<T> generator) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = EXPERIMENTAL, since = "1.8.0")
-	public static <T> Arbitrary<T> fromGeneratorWithSize(IntFunction<RandomGenerator<T>> generatorSupplier) {
+	public static <T extends @Nullable Object> Arbitrary<T> fromGeneratorWithSize(IntFunction<RandomGenerator<T>> generatorSupplier) {
 		return ArbitrariesFacade.implementation.fromGenerator(generatorSupplier);
 	}
 
@@ -123,7 +123,7 @@ public static <T> Arbitrary<T> fromGeneratorWithSize(IntFunction<RandomGenerator
 	 * @param <T>       The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <T> Arbitrary<T> randomValue(Function<Random, T> generator) {
+	public static <T extends @Nullable Object> Arbitrary<T> randomValue(Function<Random, T> generator) {
 		IntFunction<RandomGenerator<T>> generatorSupplier = ignore -> random -> Shrinkable.unshrinkable(generator.apply(random));
 		return fromGeneratorWithSize(generatorSupplier);
 	}
@@ -153,7 +153,7 @@ public static Arbitrary<Random> randoms() {
 	 * @return a new arbitrary instance
 	 */
 	@SafeVarargs
-	public static <T> Arbitrary<T> of(T... values) {
+	public static <T extends @Nullable Object> Arbitrary<T> of(T... values) {
 		return of(Arrays.asList(values));
 	}
 
@@ -172,7 +172,7 @@ public static <T> Arbitrary<T> of(T... values) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.1")
-	public static <@Nullable T> Arbitrary<T> of(Collection<T> values) {
+	public static <T extends @Nullable Object> Arbitrary<T> of(Collection<T> values) {
 		return ArbitrariesFacade.implementation.of(values);
 	}
 
@@ -192,7 +192,7 @@ public static <T> Arbitrary<T> of(T... values) {
 	 */
 	@API(status = MAINTAINED, since = "1.3.0")
 	@SafeVarargs
-	public static <T> Arbitrary<T> ofSuppliers(Supplier<T>... valueSuppliers) {
+	public static <T extends @Nullable Object> Arbitrary<T> ofSuppliers(Supplier<T>... valueSuppliers) {
 		return of(valueSuppliers).map(Supplier::get);
 	}
 
@@ -211,7 +211,7 @@ public static <T> Arbitrary<T> ofSuppliers(Supplier<T>... valueSuppliers) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.1")
-	public static <T> Arbitrary<T> ofSuppliers(Collection<Supplier<T>> valueSuppliers) {
+	public static <T extends @Nullable Object> Arbitrary<T> ofSuppliers(Collection<Supplier<T>> valueSuppliers) {
 		return of(valueSuppliers).map(Supplier::get);
 	}
 
@@ -247,7 +247,7 @@ public static <T extends Enum<T>> Arbitrary<T> of(Class<T> enumClass) {
 	 */
 	@SuppressWarnings("unchecked")
 	@SafeVarargs
-	public static <T> Arbitrary<T> oneOf(Arbitrary<? extends T> first, Arbitrary<? extends T>... rest) {
+	public static <T extends @Nullable Object> Arbitrary<T> oneOf(Arbitrary<? extends T> first, Arbitrary<? extends T>... rest) {
 		List<Arbitrary<? extends T>> all = new ArrayList<>();
 		all.add(first);
 		for (Arbitrary<?> arbitrary : rest) {
@@ -264,7 +264,7 @@ public static <T> Arbitrary<T> oneOf(Arbitrary<? extends T> first, Arbitrary<? e
 	 * @return a new arbitrary instance
 	 */
 	@SuppressWarnings("unchecked")
-	public static <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> choices) {
+	public static <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> choices) {
 		if (choices.isEmpty()) {
 			String message = "oneOf() must not be called with no choices";
 			throw new JqwikException(message);
@@ -285,7 +285,7 @@ public static <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> choices)
 	 * @return a new arbitrary instance
 	 */
 	@SafeVarargs
-	public static <T> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
 		return frequency(Arrays.asList(frequencies));
 	}
 
@@ -297,7 +297,7 @@ public static <T> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
 	 * @param <T>         The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <@Nullable T> Arbitrary<T> frequency(List<Tuple2<Integer, T>> frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequency(List<? extends Tuple2<Integer, T>> frequencies) {
 		return ArbitrariesFacade.implementation.frequency(frequencies);
 	}
 
@@ -311,7 +311,7 @@ public static <T> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
 	 */
 	@SuppressWarnings("unchecked")
 	@SafeVarargs
-	public static <T> Arbitrary<T> frequencyOf(Tuple2<Integer, Arbitrary<? extends T>>... frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequencyOf(Tuple2<Integer, Arbitrary<? extends T>>... frequencies) {
 		List<Tuple2<Integer, Arbitrary<T>>> all = new ArrayList<>();
 		for (Tuple2<Integer, Arbitrary<? extends T>> frequency : frequencies) {
 			all.add(Tuple.of(frequency.get1(), (Arbitrary<T>) frequency.get2()));
@@ -327,7 +327,7 @@ public static <T> Arbitrary<T> frequencyOf(Tuple2<Integer, Arbitrary<? extends T
 	 * @param <T>         The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <@Nullable T> Arbitrary<T> frequencyOf(List<Tuple2<Integer, Arbitrary<T>>> frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequencyOf(List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
 		// Simple flatMapping is not enough because of configurations
 		return ArbitrariesFacade.implementation.frequencyOf(frequencies);
 	}
@@ -454,7 +454,7 @@ public static CharacterArbitrary chars() {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.1.1")
-	public static <T> Arbitrary<T> create(Supplier<T> supplier) {
+	public static <T extends @Nullable Object> Arbitrary<T> create(Supplier<T> supplier) {
 		return ArbitrariesFacade.implementation.create(supplier);
 	}
 
@@ -468,7 +468,7 @@ public static <T> Arbitrary<T> create(Supplier<T> supplier) {
 	 * @return a new arbitrary instance
 	 */
 	@SafeVarargs
-	public static <T> Arbitrary<List<T>> shuffle(T... values) {
+	public static <T extends @Nullable Object> Arbitrary<List<T>> shuffle(T... values) {
 		return shuffle(Arrays.asList(values));
 	}
 
@@ -481,7 +481,7 @@ public static <T> Arbitrary<List<T>> shuffle(T... values) {
 	 * @param <T>    The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <T> Arbitrary<List<T>> shuffle(List<T> values) {
+	public static <T extends @Nullable Object> Arbitrary<List<T>> shuffle(List<T> values) {
 		return ArbitrariesFacade.implementation.shuffle(values);
 	}
 
@@ -591,7 +591,7 @@ public static <T> TraverseArbitrary<T> traverse(Class<T> targetType, Traverser t
 	 * @see #recursive(Supplier, Function, int)
 	 * @see #lazyOf(Supplier, Supplier[])
 	 */
-	public static <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier) {
+	public static <T extends @Nullable Object> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		return ArbitrariesFacade.implementation.lazy(arbitrarySupplier);
 	}
 
@@ -609,9 +609,9 @@ public static <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier) {
 	 * @return a new arbitrary instance
 	 * @see #lazy(Supplier)
 	 */
-	public static <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+	public static <T extends @Nullable Object> Arbitrary<T> recursive(
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int depth
 	) {
 		return ArbitrariesFacade.implementation.recursive(base, recur, depth, depth);
@@ -633,9 +633,9 @@ public static <T> Arbitrary<T> recursive(
 	 * @see #lazy(Supplier)
 	 */
 	@API(status = MAINTAINED, since = "1.6.4")
-	public static <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+	public static <T extends @Nullable Object> Arbitrary<T> recursive(
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int minDepth,
 		int maxDepth
 	) {
@@ -671,7 +671,7 @@ public static <T> Arbitrary<T> recursive(
 	@SuppressWarnings("unchecked")
 	@SafeVarargs
 	@API(status = MAINTAINED, since = "1.3.4")
-	public static <T> Arbitrary<T> lazyOf(Supplier<Arbitrary<? extends T>> first, Supplier<Arbitrary<? extends T>>... rest) {
+	public static <T extends @Nullable Object> Arbitrary<T> lazyOf(Supplier<Arbitrary<? extends T>> first, Supplier<Arbitrary<? extends T>>... rest) {
 		List<Supplier<Arbitrary<T>>> all = new ArrayList<>();
 		all.add(() -> (Arbitrary<T>) first.get());
 		for (Supplier<Arbitrary<? extends T>> arbitrarySupplier : rest) {
diff --git a/api/src/main/java/net/jqwik/api/Arbitrary.java b/api/src/main/java/net/jqwik/api/Arbitrary.java
index f76f8c7b4..9f0c5ed08 100644
--- a/api/src/main/java/net/jqwik/api/Arbitrary.java
+++ b/api/src/main/java/net/jqwik/api/Arbitrary.java
@@ -6,7 +6,7 @@
 
 import com.google.errorprone.annotations.*;
 import org.apiguardian.api.*;
-import org.jspecify.annotations.Nullable;
+import org.jspecify.annotations.*;
 
 import net.jqwik.api.arbitraries.*;
 
@@ -20,7 +20,7 @@
  */
 @API(status = STABLE, since = "1.0")
 @CheckReturnValue
-public interface Arbitrary<@Nullable T> {
+public interface Arbitrary<T extends @Nullable Object> {
 
 	@API(status = INTERNAL)
 	abstract class ArbitraryFacade {
diff --git a/api/src/main/java/net/jqwik/api/Builders.java b/api/src/main/java/net/jqwik/api/Builders.java
index 3a7e317d4..b96949449 100644
--- a/api/src/main/java/net/jqwik/api/Builders.java
+++ b/api/src/main/java/net/jqwik/api/Builders.java
@@ -53,7 +53,7 @@ private BuilderCombinator(Supplier<B> starter, List<Tuple3<Double, Arbitrary<Obj
 		 * @param <T>
 		 * @return new {@linkplain CombinableBuilder} instance
 		 */
-		public <@Nullable T> CombinableBuilder<B, T> use(Arbitrary<T> arbitrary) {
+		public <T extends @Nullable Object> CombinableBuilder<B, T> use(Arbitrary<T> arbitrary) {
 			return new CombinableBuilder<>(this, 1.0, arbitrary);
 		}
 
@@ -167,7 +167,7 @@ BuilderCombinator<B> withMutator(double probabilityOfUse, Arbitrary<Object> arbi
 	 *
 	 * @param <B> The builder's type
 	 */
-	public static class CombinableBuilder<B, T> {
+	public static class CombinableBuilder<B, T extends @Nullable Object> {
 		private final BuilderCombinator<B> combinator;
 		private final double probabilityOfUse;
 		private final Arbitrary<T> arbitrary;
@@ -201,7 +201,7 @@ public CombinableBuilder<B, T> withProbability(double probabilityOfUse) {
 		 * @return new {@linkplain BuilderCombinator} instance
 		 */
 		@SuppressWarnings("unchecked")
-		public BuilderCombinator<B> in(BiFunction<B, @Nullable T, B> toFunction) {
+		public BuilderCombinator<B> in(BiFunction<? super B, ? super T, ? extends B> toFunction) {
 			if (probabilityOfUse == 0.0) {
 				return combinator;
 			}
@@ -217,7 +217,7 @@ public BuilderCombinator<B> in(BiFunction<B, @Nullable T, B> toFunction) {
 		 * @param setter Use value provided by arbitrary to change a builder's property.
 		 * @return new {@linkplain BuilderCombinator} instance with same embedded builder
 		 */
-		public BuilderCombinator<B> inSetter(BiConsumer<B, T> setter) {
+		public BuilderCombinator<B> inSetter(BiConsumer<? super B, ? super T> setter) {
 			BiFunction<B, T, B> toFunction = (b, t) -> {
 				setter.accept(b, t);
 				return b;
diff --git a/api/src/main/java/net/jqwik/api/Combinators.java b/api/src/main/java/net/jqwik/api/Combinators.java
index da7011726..a8a21dfdc 100644
--- a/api/src/main/java/net/jqwik/api/Combinators.java
+++ b/api/src/main/java/net/jqwik/api/Combinators.java
@@ -68,7 +68,7 @@ public abstract <T1, T2, T3, T4, T5, T6, T7, T8> Combinator8<T1, T2, T3, T4, T5,
 			Arbitrary<T8> a8
 		);
 
-		public abstract <T> ListCombinator<T> combineList(List<Arbitrary<T>> listOfArbitraries);
+		public abstract <T> ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries);
 	}
 
 	private Combinators() {
@@ -79,7 +79,7 @@ private Combinators() {
 	 *
 	 * @return Combinator2 instance which can be evaluated using {@linkplain Combinator2#as}
 	 */
-	public static <@Nullable T1, @Nullable T2> Combinator2<T1, T2> combine(Arbitrary<T1> a1, Arbitrary<T2> a2) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object> Combinator2<T1, T2> combine(Arbitrary<T1> a1, Arbitrary<T2> a2) {
 		return CombinatorsFacade.implementation.combine2(a1, a2);
 	}
 
@@ -88,7 +88,7 @@ private Combinators() {
 	 *
 	 * @return Combinator3 instance which can be evaluated using {@linkplain Combinator3#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3> Combinator3<T1, T2, T3> combine(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> Combinator3<T1, T2, T3> combine(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3) {
 		return CombinatorsFacade.implementation.combine3(a1, a2, a3);
 	}
 
@@ -97,7 +97,7 @@ private Combinators() {
 	 *
 	 * @return Combinator4 instance which can be evaluated using {@linkplain Combinator4#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> Combinator4<T1, T2, T3, T4> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> Combinator4<T1, T2, T3, T4> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 		Arbitrary<T4> a4
 	) {
@@ -109,7 +109,7 @@ private Combinators() {
 	 *
 	 * @return Combinator5 instance which can be evaluated using {@linkplain Combinator5#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> Combinator5<T1, T2, T3, T4, T5> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> Combinator5<T1, T2, T3, T4, T5> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 		Arbitrary<T4> a4, Arbitrary<T5> a5
 	) {
@@ -121,7 +121,7 @@ private Combinators() {
 	 *
 	 * @return Combinator6 instance which can be evaluated using {@linkplain Combinator6#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> Combinator6<T1, T2, T3, T4, T5, T6> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> Combinator6<T1, T2, T3, T4, T5, T6> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 		Arbitrary<T4> a4, Arbitrary<T5> a5, Arbitrary<T6> a6
 	) {
@@ -133,7 +133,7 @@ private Combinators() {
 	 *
 	 * @return Combinator7 instance which can be evaluated using {@linkplain Combinator7#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> Combinator7<T1, T2, T3, T4, T5, T6, T7> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> Combinator7<T1, T2, T3, T4, T5, T6, T7> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2,
 		Arbitrary<T3> a3, Arbitrary<T4> a4, Arbitrary<T5> a5, Arbitrary<T6> a6, Arbitrary<T7> a7
 	) {
@@ -145,7 +145,7 @@ private Combinators() {
 	 *
 	 * @return Combinator8 instance which can be evaluated using {@linkplain Combinator8#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2,
 		Arbitrary<T3> a3, Arbitrary<T4> a4, Arbitrary<T5> a5, Arbitrary<T6> a6, Arbitrary<T7> a7, Arbitrary<T8> a8
 	) {
@@ -157,14 +157,14 @@ private Combinators() {
 	 *
 	 * @return ListCombinator instance which can be evaluated using {@linkplain ListCombinator#as}
 	 */
-	public static <@Nullable T> ListCombinator<T> combine(List<Arbitrary<T>> listOfArbitraries) {
+	public static <T extends @Nullable Object> ListCombinator<T> combine(List<? extends Arbitrary<T>> listOfArbitraries) {
 		return CombinatorsFacade.implementation.combineList(listOfArbitraries);
 	}
 
 	/**
 	 * Combinator for two values.
 	 */
-	public interface Combinator2<@Nullable T1, @Nullable T2> {
+	public interface Combinator2<T1 extends @Nullable Object, T2 extends @Nullable Object> {
 
 		/**
 		 * Combine two values.
@@ -173,7 +173,7 @@ public interface Combinator2<@Nullable T1, @Nullable T2> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F2<T1, T2, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F2<T1, T2, R> combinator);
 
 		/**
 		 * Filter two values to only let them pass if the predicate is true.
@@ -191,7 +191,7 @@ public interface Combinator2<@Nullable T1, @Nullable T2> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F2<T1, T2, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F2<T1, T2, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -199,7 +199,7 @@ public interface Combinator2<@Nullable T1, @Nullable T2> {
 	/**
 	 * Combinator for three values.
 	 */
-	public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
+	public interface Combinator3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> {
 
 		/**
 		 * Combine three values.
@@ -208,7 +208,7 @@ public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F3<T1, T2, T3, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F3<T1, T2, T3, R> combinator);
 
 		/**
 		 * Filter three values to only let them pass if the predicate is true.
@@ -226,7 +226,7 @@ public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F3<T1, T2, T3, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F3<T1, T2, T3, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -235,7 +235,7 @@ public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
 	/**
 	 * Combinator for four values.
 	 */
-	public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> {
+	public interface Combinator4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> {
 
 		/**
 		 * Combine four values.
@@ -244,7 +244,7 @@ public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F4<T1, T2, T3, T4, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F4<T1, T2, T3, T4, R> combinator);
 
 		/**
 		 * Filter four values to only let them pass if the predicate is true.
@@ -262,7 +262,7 @@ public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F4<T1, T2, T3, T4, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F4<T1, T2, T3, T4, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -271,7 +271,7 @@ public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for five values.
 	 */
-	public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> {
+	public interface Combinator5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> {
 
 		/**
 		 * Combine five values.
@@ -280,7 +280,7 @@ public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F5<T1, T2, T3, T4, T5, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F5<T1, T2, T3, T4, T5, R> combinator);
 
 		/**
 		 * Filter five values to only let them pass if the predicate is true.
@@ -298,7 +298,7 @@ public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F5<T1, T2, T3, T4, T5, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F5<T1, T2, T3, T4, T5, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -306,7 +306,7 @@ public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for six values.
 	 */
-	public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> {
+	public interface Combinator6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> {
 
 		/**
 		 * Combine six values.
@@ -315,7 +315,7 @@ public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F6<T1, T2, T3, T4, T5, T6, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F6<T1, T2, T3, T4, T5, T6, R> combinator);
 
 		/**
 		 * Filter six values to only let them pass if the predicate is true.
@@ -333,7 +333,7 @@ public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F6<T1, T2, T3, T4, T5, T6, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F6<T1, T2, T3, T4, T5, T6, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -342,7 +342,7 @@ public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for seven values.
 	 */
-	public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> {
+	public interface Combinator7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> {
 
 		/**
 		 * Combine seven values.
@@ -351,7 +351,7 @@ public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F7<T1, T2, T3, T4, T5, T6, T7, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F7<T1, T2, T3, T4, T5, T6, T7, R> combinator);
 
 		/**
 		 * Filter seven values to only let them pass if the predicate is true.
@@ -369,7 +369,7 @@ public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F7<T1, T2, T3, T4, T5, T6, T7, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F7<T1, T2, T3, T4, T5, T6, T7, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -378,7 +378,7 @@ public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for eight values.
 	 */
-	public interface Combinator8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> {
+	public interface Combinator8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> {
 
 		/**
 		 * Combine eight values.
@@ -387,7 +387,7 @@ public interface Combinator8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator);
 
 		/**
 		 * Filter eight values to only let them pass if the predicate is true.
@@ -405,7 +405,7 @@ public interface Combinator8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F8<T1, T2, T3, T4, T5, T6, T7, T8, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F8<T1, T2, T3, T4, T5, T6, T7, T8, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -422,7 +422,7 @@ public interface ListCombinator<@Nullable T> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(Function<List<T>, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(Function<List<T>, R> combinator);
 
 		/**
 		 * Filter list of values to only let them pass if the predicate is true.
@@ -440,50 +440,50 @@ public interface ListCombinator<@Nullable T> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(Function<List<T>, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(Function<List<T>, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F2<@Nullable T1, @Nullable T2, @Nullable R> {
+	public interface F2<T1 extends @Nullable Object, T2 extends @Nullable Object, @Nullable R> {
 		R apply(T1 t1, T2 t2);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F3<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable R> {
+	public interface F3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, @Nullable R> {
 		R apply(T1 t1, T2 t2, T3 t3);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable R> {
+	public interface F4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, @Nullable R> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable R> {
+	public interface F5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable R> {
+	public interface F6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5, T6 t6);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable R> {
+	public interface F7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5, T6 t6, T7 t7);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8, @Nullable R> {
+	public interface F8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5, T6 t6, T7 t7, T8 t8);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/RandomGenerator.java b/api/src/main/java/net/jqwik/api/RandomGenerator.java
index 694d66742..b47f3871e 100644
--- a/api/src/main/java/net/jqwik/api/RandomGenerator.java
+++ b/api/src/main/java/net/jqwik/api/RandomGenerator.java
@@ -5,11 +5,12 @@
 import java.util.stream.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = STABLE, since = "1.0")
-public interface RandomGenerator<T> {
+public interface RandomGenerator<T extends @Nullable Object> {
 
 	@API(status = INTERNAL)
 	abstract class RandomGeneratorFacade {
diff --git a/api/src/main/java/net/jqwik/api/Shrinkable.java b/api/src/main/java/net/jqwik/api/Shrinkable.java
index c99d9bd3c..c7a4c87db 100644
--- a/api/src/main/java/net/jqwik/api/Shrinkable.java
+++ b/api/src/main/java/net/jqwik/api/Shrinkable.java
@@ -10,7 +10,7 @@
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = STABLE, since = "1.0")
-public interface Shrinkable<T> extends Comparable<Shrinkable<T>> {
+public interface Shrinkable<T extends @Nullable Object> extends Comparable<Shrinkable<T>> {
 
 	@API(status = INTERNAL)
 	abstract class ShrinkableFacade {
diff --git a/api/src/main/java/net/jqwik/api/Tuple.java b/api/src/main/java/net/jqwik/api/Tuple.java
index 438d36e99..6b97d3cc1 100644
--- a/api/src/main/java/net/jqwik/api/Tuple.java
+++ b/api/src/main/java/net/jqwik/api/Tuple.java
@@ -37,23 +37,23 @@ static Tuple0 empty() {
 		return Tuple.of();
 	}
 
-	static <@Nullable T1> Tuple1<T1> of(T1 v1) {
+	static <T1 extends @Nullable Object> Tuple1<T1> of(T1 v1) {
 		return new Tuple1<>(v1);
 	}
 
-	static <@Nullable T1, @Nullable T2> Tuple2<T1, T2> of(T1 v1, T2 v2) {
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object> Tuple2<T1, T2> of(T1 v1, T2 v2) {
 		return new Tuple2<>(v1, v2);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3> Tuple3<T1, T2, T3> of(T1 v1, T2 v2, T3 v3) {
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> Tuple3<T1, T2, T3> of(T1 v1, T2 v2, T3 v3) {
 		return new Tuple3<>(v1, v2, v3);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> Tuple4<T1, T2, T3, T4> of(T1 v1, T2 v2, T3 v3, T4 v4) {
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> Tuple4<T1, T2, T3, T4> of(T1 v1, T2 v2, T3 v3, T4 v4) {
 		return new Tuple4<>(v1, v2, v3, v4);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> Tuple5<T1, T2, T3, T4, T5> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> Tuple5<T1, T2, T3, T4, T5> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -63,7 +63,7 @@ static Tuple0 empty() {
 		return new Tuple5<>(v1, v2, v3, v4, v5);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> Tuple6<T1, T2, T3, T4, T5, T6> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> Tuple6<T1, T2, T3, T4, T5, T6> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -74,7 +74,7 @@ static Tuple0 empty() {
 		return new Tuple6<>(v1, v2, v3, v4, v5, v6);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> Tuple7<T1, T2, T3, T4, T5, T6, T7> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> Tuple7<T1, T2, T3, T4, T5, T6, T7> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -86,7 +86,7 @@ static Tuple0 empty() {
 		return new Tuple7<>(v1, v2, v3, v4, v5, v6, v7);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> Tuple8<T1, T2, T3, T4, T5, T6, T7, T8> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> Tuple8<T1, T2, T3, T4, T5, T6, T7, T8> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -122,10 +122,10 @@ public String toString() {
 		}
 	}
 
-	class Tuple1<@Nullable T1> extends Tuple0 {
+	class Tuple1<T1 extends @Nullable Object> extends Tuple0 {
 		final T1 v1;
 
-		private Tuple1(@Nullable T1 v1) {
+		private Tuple1(T1 v1) {
 			super();
 			this.v1 = v1;
 		}
@@ -163,7 +163,7 @@ public String toString() {
 		}
 	}
 
-	class Tuple2<@Nullable T1, @Nullable T2> extends Tuple1<T1> {
+	class Tuple2<T1 extends @Nullable Object, T2 extends @Nullable Object> extends Tuple1<T1> {
 		final T2 v2;
 
 		private Tuple2(T1 v1, T2 v2) {
@@ -200,7 +200,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple3<@Nullable T1, @Nullable T2, @Nullable T3> extends Tuple2<T1, T2> {
+	class Tuple3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> extends Tuple2<T1, T2> {
 		final T3 v3;
 
 		private Tuple3(T1 v1, T2 v2, T3 v3) {
@@ -238,7 +238,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> extends Tuple3<T1, T2, T3> {
+	class Tuple4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> extends Tuple3<T1, T2, T3> {
 		final T4 v4;
 
 		private Tuple4(T1 v1, T2 v2, T3 v3, T4 v4) {
@@ -277,7 +277,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> extends Tuple4<T1, T2, T3, T4> {
+	class Tuple5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> extends Tuple4<T1, T2, T3, T4> {
 		final T5 v5;
 
 		private Tuple5(T1 v1, T2 v2, T3 v3, T4 v4, T5 v5) {
@@ -317,7 +317,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> extends Tuple5<T1, T2, T3, T4, T5> {
+	class Tuple6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> extends Tuple5<T1, T2, T3, T4, T5> {
 		final T6 v6;
 
 		private Tuple6(T1 v1, T2 v2, T3 v3, T4 v4, T5 v5, T6 v6) {
@@ -358,7 +358,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> extends Tuple6<T1, T2, T3, T4, T5, T6> {
+	class Tuple7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> extends Tuple6<T1, T2, T3, T4, T5, T6> {
 		final T7 v7;
 
 		private Tuple7(
@@ -408,7 +408,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> extends Tuple7<T1, T2, T3, T4, T5, T6, T7> {
+	class Tuple8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> extends Tuple7<T1, T2, T3, T4, T5, T6, T7> {
 		final T8 v8;
 
 		private Tuple8(
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java
index 7a93878bf..cee278c1b 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java
@@ -12,9 +12,11 @@
 /**
  * Fluent interface to add functionality to arbitraries that generate instances
  * of Arrays
+ *
+ * @param <A> type of the resulting array class
  */
 @API(status = MAINTAINED, since = "1.4.0")
-public interface ArrayArbitrary<@Nullable T, A> extends StreamableArbitrary<T, A> {
+public interface ArrayArbitrary<T extends @Nullable Object, A> extends StreamableArbitrary<T, A> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -76,6 +78,6 @@ default ArrayArbitrary<T, A> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	ArrayArbitrary<T, A> uniqueElements(Function<T, Object> by);
+	ArrayArbitrary<T, A> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
index 85bec6760..808963692 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
@@ -17,7 +17,7 @@
  * @param <R> The return type of the functional interface
  */
 @API(status = MAINTAINED, since = "1.3.0")
-public interface FunctionArbitrary<F, @Nullable R> extends Arbitrary<F> {
+public interface FunctionArbitrary<F extends @Nullable Object, R extends @Nullable Object> extends Arbitrary<F> {
 
 	/**
 	 *
@@ -28,6 +28,6 @@ public interface FunctionArbitrary<F, @Nullable R> extends Arbitrary<F> {
 	 * @param answer A function that produces a concrete answer or throws an exception
 	 * @return A new instance of function arbitrary
 	 */
-	<F_ extends F> FunctionArbitrary<F_, R> when(Predicate<List<Object>> parameterCondition, Function<List<Object>, R> answer);
+	<F_ extends F> FunctionArbitrary<F_, R> when(Predicate<? super List<?>> parameterCondition, Function<? super List<?>, ? extends R> answer);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java
index d854a02b4..0d6d8e16c 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Iterator}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface IteratorArbitrary<T> extends Arbitrary<Iterator<T>>, StreamableArbitrary<T, Iterator<T>> {
+public interface IteratorArbitrary<T extends @Nullable Object> extends Arbitrary<Iterator<T>>, StreamableArbitrary<T, Iterator<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -76,6 +78,6 @@ default IteratorArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	IteratorArbitrary<T> uniqueElements(Function<T, Object> by);
+	IteratorArbitrary<T> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
index ed7ae6a8e..4c2e69655 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
@@ -16,7 +16,7 @@
  * of type {@linkplain List}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface ListArbitrary<T> extends Arbitrary<List<T>>, StreamableArbitrary<T, List<T>> {
+public interface ListArbitrary<T extends @Nullable Object> extends Arbitrary<List<T>>, StreamableArbitrary<T, List<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -72,7 +72,7 @@ default ListArbitrary<T> ofSize(int size) {
 	 * @return arbitrary of a list of Us
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	<U> Arbitrary<List<U>> flatMapEach(BiFunction<List<T>, T, Arbitrary<U>> flatMapper);
+	<U> Arbitrary<List<U>> flatMapEach(BiFunction<? super List<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper);
 
 	/**
 	 * Add the constraint that elements of the generated list must be unique,
@@ -100,5 +100,5 @@ default ListArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	ListArbitrary<@Nullable T> uniqueElements(Function<@Nullable T, Object> by);
+	ListArbitrary<T> uniqueElements(Function<? super T, ?> by);
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java
index fb0ec32c4..1f9ef22dc 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Map}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface MapArbitrary<K, V> extends Arbitrary<Map<K, V>>, SizableArbitrary<Map<K, V>> {
+public interface MapArbitrary<K extends @Nullable Object, V extends @Nullable Object> extends Arbitrary<Map<K, V>>, SizableArbitrary<Map<K, V>> {
 
 	/**
 	 * Fix the size to {@code size}.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
index d8a66e983..f9a9fe1b5 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Set}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface SetArbitrary<T> extends Arbitrary<Set<T>>, StreamableArbitrary<T, Set<T>> {
+public interface SetArbitrary<T extends @Nullable Object> extends Arbitrary<Set<T>>, StreamableArbitrary<T, Set<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -94,6 +96,6 @@ default SetArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	SetArbitrary<T> uniqueElements(Function<T, Object> by);
+	SetArbitrary<T> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java
index 0035cab95..8f270a035 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java
@@ -4,13 +4,15 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
  * Fluent interface to configure arbitraries that have size constraints for generated values, e.g. collections and arrays.
  */
 @API(status = MAINTAINED, since = "1.0")
-public interface SizableArbitrary<U> extends Arbitrary<U> {
+public interface SizableArbitrary<U extends @Nullable Object> extends Arbitrary<U> {
 
 	/**
 	 * Fix the size to {@code size}.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java
index 1c5d93d3c..c200f6b37 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Stream}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface StreamArbitrary<T> extends Arbitrary<Stream<T>>, StreamableArbitrary<T, Stream<T>> {
+public interface StreamArbitrary<T extends @Nullable Object> extends Arbitrary<Stream<T>>, StreamableArbitrary<T, Stream<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -76,6 +78,6 @@ default StreamArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	StreamArbitrary<T> uniqueElements(Function<T, Object> by);
+	StreamArbitrary<T> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
index 4f8c70f52..d45d63565 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
@@ -17,7 +17,7 @@
  * can be streamed, e.g. {@link List}, {@link Set}, {@link Stream} and Arrays
  */
 @API(status = MAINTAINED, since = "1.2.1")
-public interface StreamableArbitrary<T, U> extends SizableArbitrary<U> {
+public interface StreamableArbitrary<T extends @Nullable Object, U> extends SizableArbitrary<U> {
 
 	/**
 	 * Given an {@code initial} argument use {@code accumulator} to produce
@@ -79,6 +79,6 @@ default StreamableArbitrary<T, U> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.7.3")
-	StreamableArbitrary<@Nullable T, U> uniqueElements(Function<@Nullable T, Object> by);
+	StreamableArbitrary<T, U> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java
index adc5cfe64..5a7ec3c20 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java
@@ -8,6 +8,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.providers.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -15,7 +17,7 @@
  * of a given type {@code T} from the type's available constructors and factory methods.
  */
 @API(status = MAINTAINED, since = "1.8.0")
-public interface TraverseArbitrary<T> extends Arbitrary<T> {
+public interface TraverseArbitrary<T extends @Nullable Object> extends Arbitrary<T> {
 
 	/**
 	 * A traverser describes how to travers a given type by providing a hook to
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
index d5101fc32..0ea63796a 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -24,7 +26,7 @@
  * @see net.jqwik.api.domains.DomainContext
  */
 @API(status = MAINTAINED, since = "1.2")
-public interface TypeArbitrary<T> extends Arbitrary<T> {
+public interface TypeArbitrary<T extends @Nullable Object> extends Arbitrary<T> {
 
 	/**
 	 * Add public constructors of class {@code T} to be used
diff --git a/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java b/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
index e841d5978..03856d196 100644
--- a/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
+++ b/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
@@ -80,7 +80,7 @@ interface CoverageChecker {
 	 * @param query A {@link Predicate} that takes the collected values as parameter
 	 *              and returns true if the specific values shall be counted.
 	 */
-	CoverageChecker checkQuery(Predicate<? extends List<?>> query);
+	CoverageChecker checkQuery(Predicate<? super List<?>> query);
 
 	/**
 	 * Match collected values against a regular expression.
diff --git a/documentation/src/docs/include/customized-parameter-generation.md b/documentation/src/docs/include/customized-parameter-generation.md
index 4970f3270..264bf2a8c 100644
--- a/documentation/src/docs/include/customized-parameter-generation.md
+++ b/documentation/src/docs/include/customized-parameter-generation.md
@@ -1051,14 +1051,14 @@ either [annotating parameters with `@UniqueElements`](#unique-elements)
 or by using one of the many `uniqueness(..)` configuration methods for 
 collections and collection-like types:
 
-- `ListArbitrary<T>.uniqueElements(Function<T, Object>)`
+- `ListArbitrary<T>.uniqueElements(Function<? super T, ?>)`
 - `ListArbitrary<T>.uniqueElements()`
-- `SetArbitrary<T>.uniqueElements(Function<T, Object>)`
-- `StreamArbitrary<T>.uniqueElements(Function<T, Object>)`
+- `SetArbitrary<T>.uniqueElements(Function<? super T, ?>)`
+- `StreamArbitrary<T>.uniqueElements(Function<? super T, ?>)`
 - `StreamArbitrary<T>.uniqueElements()`
-- `IteratorArbitrary<T>.uniqueElements(Function<T, Object>)`
+- `IteratorArbitrary<T>.uniqueElements(Function<? super T, ?>)`
 - `IteratorArbitrary<T>.uniqueElements()`
-- `ArrayArbitrary<T, A>.uniqueElements(Function<T, Object>)`
+- `ArrayArbitrary<T, A>.uniqueElements(Function<? super T, ?>)`
 - `ArrayArbitrary<T, A>.uniqueElements()`
 - `MapArbitrary<K, V>.uniqueKeys(Function<K, Object>)`
 - `MapArbitrary<K, V>.uniqueValues(Function<V, Object>)`
diff --git a/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java b/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java
index 36762d99d..298371e65 100644
--- a/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java
+++ b/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java
@@ -57,7 +57,10 @@ void queryStatistics(@ForAll int anInt) {
 		Statistics.collect(anInt);
 
 		Statistics.coverage(coverage -> {
-			Predicate<List<Integer>> isZero = params -> params.get(0) == 0;
+			// Note: coverage.checkQuery is untuped as it requires Predicate that must support **any** object
+			// So we can't have Predicate<List<Integer>> here as checkQuery would not be able to call our
+			// predicate with Object values.
+			Predicate<List<?>> isZero = params -> (Integer) params.get(0) == 0;
 			coverage.checkQuery(isZero).percentage(p -> p > 5.0);
 		});
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
index 00eab6625..f4a318285 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
@@ -16,6 +16,8 @@
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 import net.jqwik.engine.properties.stateful.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.properties.arbitraries.ArbitrariesSupport.*;
 
 /**
@@ -24,12 +26,12 @@
 public class ArbitrariesFacadeImpl extends Arbitraries.ArbitrariesFacade {
 
 	@Override
-	public <T> Arbitrary<T> just(T value) {
+	public <T extends @Nullable Object> Arbitrary<T> just(T value) {
 		return new JustArbitrary<>(value);
 	}
 
 	@Override
-	public <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> choices) {
+	public <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> choices) {
 		return new OneOfArbitrary<>(choices);
 	}
 
@@ -39,8 +41,8 @@ public <M> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> a
 	}
 
 	@Override
-	public <T> Arbitrary<T> frequencyOf(List<Tuple.Tuple2<Integer, Arbitrary<T>>> frequencies) {
-		List<Tuple.Tuple2<Integer, Arbitrary<T>>> aboveZeroFrequencies =
+	public <T> Arbitrary<T> frequencyOf(List<? extends Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
+		List<Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> aboveZeroFrequencies =
 			frequencies.stream().filter(f -> f.get1() > 0).collect(Collectors.toList());
 
 		if (aboveZeroFrequencies.size() == 1) {
@@ -105,12 +107,12 @@ public CharacterArbitrary chars() {
 	}
 
 	@Override
-	public <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier) {
+	public <T> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		return new LazyArbitrary<>(arbitrarySupplier);
 	}
 
 	@Override
-	public <T> Arbitrary<T> lazyOf(List<Supplier<Arbitrary<T>>> suppliers) {
+	public <T> Arbitrary<T> lazyOf(List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		int hashIdentifier = calculateIdentifier(suppliers.size());
 		return LazyOfArbitrary.of(hashIdentifier, suppliers);
 	}
@@ -142,12 +144,12 @@ public <T> Arbitrary<List<T>> shuffle(List<T> values) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> fromGenerator(IntFunction<RandomGenerator<T>> generatorSupplier) {
+	public <T> Arbitrary<T> fromGenerator(IntFunction<? extends RandomGenerator<T>> generatorSupplier) {
 		return new FromGeneratorWithSizeArbitrary<>(generatorSupplier);
 	}
 
 	@Override
-	public <T> Arbitrary<T> frequency(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	public <T> Arbitrary<T> frequency(List<? extends Tuple.Tuple2<Integer, T>> frequencies) {
 		List<Tuple.Tuple2<Integer, T>> frequenciesAbove0 = frequencies.stream()
 																	  .filter(f -> f.get1() > 0)
 																	  .collect(Collectors.toList());
@@ -259,9 +261,9 @@ private static Set<Arbitrary<?>> createDefaultArbitraries(
 	}
 
 	@Override
-	public <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+	public <T extends @Nullable Object> Arbitrary<T> recursive(
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int minDepth,
 		int maxDepth
 	) {
@@ -285,8 +287,8 @@ public <T> Arbitrary<T> recursive(
 	}
 
 	private <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int depth
 	) {
 		return new RecursiveArbitrary<>(base, recur, depth);
diff --git a/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
index d11d16de1..27b8dfaa0 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
@@ -81,7 +81,7 @@ public <T1, T2, T3, T4, T5, T6, T7, T8> Combinators.Combinator8<T1, T2, T3, T4,
 	}
 
 	@Override
-	public <T> Combinators.ListCombinator<T> combineList(List<Arbitrary<T>> listOfArbitraries) {
+	public <T> Combinators.ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries) {
 		return new DefaultListCombinator<>(listOfArbitraries);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java b/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
index 95b5725b6..cee4275a6 100644
--- a/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
@@ -80,7 +80,7 @@ private StatisticsEntry statisticsEntry(Object[] values) {
 				   .orElse(StatisticsEntryImpl.nullFor(key));
 	}
 
-	private StatisticsEntry query(Predicate<List<Object>> query, int countAll) {
+	private StatisticsEntry query(Predicate<? super List<?>> query, int countAll) {
 		return statisticsEntries()
 				   .stream()
 				   .filter(entry -> {
@@ -198,9 +198,9 @@ public CoverageChecker check(Object... values) {
 
 		@SuppressWarnings("unchecked")
 		@Override
-		public CoverageChecker checkQuery(Predicate<? extends List<?>> query) {
+		public CoverageChecker checkQuery(Predicate<? super List<?>> query) {
 			int countAll = countAllCollects();
-			StatisticsEntry entry = query((Predicate<List<Object>>) query, countAll);
+			StatisticsEntry entry = query(query, countAll);
 			return new CoverageCheckerImpl(label, entry, countAll);
 		}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
index 0f6e55912..4b7fadece 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
@@ -95,7 +95,7 @@ public ArrayArbitrary<T, A> uniqueElements() {
 	}
 
 	@Override
-	public ArrayArbitrary<T, A> uniqueElements(Function<T, Object> by) {
+	public ArrayArbitrary<T, A> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (ArrayArbitrary<T, A>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java
index a275971e0..e21459f08 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java
@@ -13,7 +13,7 @@ public class DefaultFunctionArbitrary<F, R> extends TypedCloneable implements Fu
 
 	private final Class<F> functionalType;
 	private final Arbitrary<R> resultArbitrary;
-	private final List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions = new ArrayList<>();
+	private final List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions = new ArrayList<>();
 
 	public DefaultFunctionArbitrary(Class<F> functionalType, Arbitrary<R> resultArbitrary) {
 		this.functionalType = functionalType;
@@ -52,15 +52,15 @@ public boolean equals(Object o) {
 	}
 
 	private boolean conditionsAreEqual(
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> left,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> right
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> left,
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> right
 	) {
 		if (left.size() != right.size()) {
 			return false;
 		}
 		for (int i = 0; i < left.size(); i++) {
-			Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> leftCondition = left.get(i);
-			Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> rightCondition = right.get(i);
+			Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> leftCondition = left.get(i);
+			Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> rightCondition = right.get(i);
 			if (!LambdaSupport.areEqual(leftCondition.get1(), rightCondition.get1())) {
 				return false;
 			}
@@ -94,14 +94,14 @@ private ConstantFunctionGenerator<F, R> createConstantFunctionGenerator(final in
 	}
 
 	@Override
-	public <F_ extends F> FunctionArbitrary<F_, R> when(Predicate<List<Object>> parameterCondition, Function<List<Object>, R> answer) {
+	public <F_ extends F> FunctionArbitrary<F_, R> when(Predicate<? super List<?>> parameterCondition, Function<? super List<?>, ? extends R> answer) {
 		DefaultFunctionArbitrary<F_, R> clone = typedClone();
 		clone.conditions.addAll(this.conditions);
 		clone.addCondition(Tuple.of(parameterCondition, answer));
 		return clone;
 	}
 
-	private void addCondition(Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> condition) {
+	private void addCondition(Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> condition) {
 		conditions.add(condition);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
index ffef597f4..cd14bc789 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
@@ -60,7 +60,7 @@ public IteratorArbitrary<T> withSizeDistribution(RandomDistribution distribution
 	}
 
 	@Override
-	public IteratorArbitrary<T> uniqueElements(Function<T, Object> by) {
+	public IteratorArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (IteratorArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
index 0691b3587..389031013 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
@@ -12,7 +12,7 @@
 
 import org.jspecify.annotations.*;
 
-public class DefaultListArbitrary<T> extends MultivalueArbitraryBase<T, List<T>> implements ListArbitrary<T> {
+public class DefaultListArbitrary<T extends @Nullable Object> extends MultivalueArbitraryBase<T, List<T>> implements ListArbitrary<T> {
 
 	public DefaultListArbitrary(Arbitrary<T> elementArbitrary) {
 		super(elementArbitrary);
@@ -68,9 +68,9 @@ public <U> Arbitrary<List<U>> mapEach(BiFunction<List<T>, T, U> mapper) {
 
 	// TODO: Remove duplication with DefaultSetArbitrary.flatMapEach()
 	@Override
-	public <U> Arbitrary<List<U>> flatMapEach(BiFunction<List<T>, T, Arbitrary<U>> flatMapper) {
+	public <U> Arbitrary<List<U>> flatMapEach(BiFunction<? super List<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper) {
 		return this.flatMap(elements -> {
-			List<Arbitrary<U>> arbitraries =
+			List<? extends Arbitrary<U>> arbitraries =
 				elements.stream()
 						.map(e -> flatMapper.apply(elements, e))
 						.collect(Collectors.toList());
@@ -79,7 +79,7 @@ public <U> Arbitrary<List<U>> flatMapEach(BiFunction<List<T>, T, Arbitrary<U>> f
 	}
 
 	@Override
-	public ListArbitrary<@Nullable T> uniqueElements(Function<@Nullable T, Object> by) {
+	public ListArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (ListArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
index ad48dd029..5ede6e77f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
@@ -93,7 +93,7 @@ public SetArbitrary<T> uniqueElements() {
 	}
 
 	@Override
-	public SetArbitrary<T> uniqueElements(Function<T, Object> by) {
+	public SetArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (SetArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
index fa289d647..c706cdb5a 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
@@ -62,7 +62,7 @@ public StreamArbitrary<T> withSizeDistribution(RandomDistribution distribution)
 	}
 
 	@Override
-	public StreamArbitrary<T> uniqueElements(Function<T, Object> by) {
+	public StreamArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (StreamArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
index 2bb22e671..1f0ad50c9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
@@ -52,7 +52,7 @@ public static <T> EdgeCases<T> choose(final List<T> values, int maxEdgeCases) {
 		return EdgeCasesSupport.fromShrinkables(shrinkables);
 	}
 
-	public static <T> EdgeCases<T> concatFrom(final List<Arbitrary<T>> arbitraries, int maxEdgeCases) {
+	public static <T> EdgeCases<T> concatFrom(final List<? extends Arbitrary<T>> arbitraries, int maxEdgeCases) {
 		List<Shrinkable<Arbitrary<T>>> shrinkables = new ArrayList<>();
 		for (Arbitrary<T> arbitrary : arbitraries) {
 			shrinkables.add(new ChooseValueShrinkable<>(arbitrary, arbitraries));
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
index 4c884eca7..2fb69bcc5 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
@@ -13,10 +13,10 @@
 
 public class FrequencyOfArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
 
-	private final List<Tuple2<Integer, Arbitrary<T>>> frequencies;
+	private final List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies;
 	private final boolean isGeneratorMemoizable;
 
-	public FrequencyOfArbitrary(List<Tuple2<Integer, Arbitrary<T>>> frequencies) {
+	public FrequencyOfArbitrary(List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
 		this.frequencies = frequencies;
 		this.isGeneratorMemoizable = frequencies.stream().allMatch(t -> t.get2().isGeneratorMemoizable());
 		if (this.frequencies.isEmpty()) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
index 9497e236d..38971e03b 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
@@ -7,9 +7,9 @@
 
 public class FromGeneratorWithSizeArbitrary<T> implements Arbitrary<T> {
 
-	private final IntFunction<RandomGenerator<T>> supplier;
+	private final IntFunction<? extends RandomGenerator<T>> supplier;
 
-	public FromGeneratorWithSizeArbitrary(IntFunction<RandomGenerator<T>> generatorSupplier) {
+	public FromGeneratorWithSizeArbitrary(IntFunction<? extends RandomGenerator<T>> generatorSupplier) {
 		this.supplier = generatorSupplier;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
index 49dd53c5a..9da910f76 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
@@ -8,11 +8,11 @@
 import net.jqwik.api.providers.*;
 
 public class LazyArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
-	private final Supplier<Arbitrary<T>> arbitrarySupplier;
+	private final Supplier<? extends Arbitrary<T>> arbitrarySupplier;
 	private final List<Tuple.Tuple2<ArbitraryConfigurator, TypeUsage>> configurations = new ArrayList<>();
 	private Arbitrary<T> arbitrary;
 
-	public LazyArbitrary(Supplier<Arbitrary<T>> arbitrarySupplier) {
+	public LazyArbitrary(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		this.arbitrarySupplier = arbitrarySupplier;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
index 4cd3798f1..9afd129f8 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
@@ -24,13 +24,13 @@ private static Store<Map<Integer, LazyOfArbitrary<?>>> arbitrariesStore() {
 	}
 
 	@SuppressWarnings("unchecked")
-	public static <T> Arbitrary<T> of(int hashIdentifier, List<Supplier<Arbitrary<T>>> suppliers) {
+	public static <T> Arbitrary<T> of(int hashIdentifier, List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		// It's important for good shrinking to work that the same arbitrary usage is handled by the same arbitrary instance
 		LazyOfArbitrary<?> arbitrary = arbitrariesStore().get().computeIfAbsent(hashIdentifier, ignore -> new LazyOfArbitrary<>(suppliers));
 		return (Arbitrary<T>) arbitrary;
 	}
 
-	private final List<Supplier<Arbitrary<T>>> suppliers;
+	private final List<? extends Supplier<? extends Arbitrary<T>>> suppliers;
 
 	private final Deque<Set<LazyOfShrinkable<T>>> generatedParts = new ArrayDeque<>();
 
@@ -45,7 +45,7 @@ private Store<Map<Integer, RandomGenerator<T>>> createGeneratorsStore() {
 		}
 	}
 
-	public LazyOfArbitrary(List<Supplier<Arbitrary<T>>> suppliers) {
+	public LazyOfArbitrary(List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		this.suppliers = suppliers;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
index 8c3eacb7b..a928246f6 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
@@ -16,8 +16,9 @@ public class OneOfArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary
 	private final boolean isGeneratorMemoizable;
 
 	@SuppressWarnings("unchecked")
-	public OneOfArbitrary(Collection<Arbitrary<? extends T>> choices) {
+	public OneOfArbitrary(Collection<? extends Arbitrary<? extends T>> choices) {
 		for (Arbitrary<? extends T> choice : choices) {
+			// TODO: double-check cast
 			all.add((Arbitrary<T>) choice);
 		}
 		isGeneratorMemoizable = all.stream().allMatch(Arbitrary::isGeneratorMemoizable);
@@ -39,7 +40,7 @@ public boolean isGeneratorMemoizable() {
 	}
 
 	private RandomGenerator<T> rawGeneration(int genSize, boolean withEmbeddedEdgeCases) {
-		List<Tuple2<Integer, Arbitrary<T>>> frequencies =
+		List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies =
 			all.stream()
 			   .map(a -> Tuple.of(1, a))
 			   .collect(Collectors.toList());
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
index c713a968b..1494c3b00 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
@@ -7,8 +7,8 @@
 import net.jqwik.api.support.*;
 
 public class RecursiveArbitrary<T> implements Arbitrary<T> {
-	private final Supplier<Arbitrary<T>> base;
-	private final Function<Arbitrary<T>, Arbitrary<T>> recur;
+	private final Supplier<? extends Arbitrary<T>> base;
+	private final Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur;
 	private final int depth;
 
 	// Not used for exhaustive generation
@@ -16,7 +16,7 @@ public class RecursiveArbitrary<T> implements Arbitrary<T> {
 
 	private boolean isGeneratorMemoizable = true;
 
-	public RecursiveArbitrary(Supplier<Arbitrary<T>> base, Function<Arbitrary<T>, Arbitrary<T>> recur, int depth) {
+	public RecursiveArbitrary(Supplier<? extends Arbitrary<T>> base, Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur, int depth) {
 		this.base = base;
 		this.recur = recur;
 		this.depth = depth;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
index 8d4c84f5d..5c661c4f7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
@@ -15,7 +15,7 @@ public class DefaultListCombinator<T> implements Combinators.ListCombinator<T> {
 	protected final Arbitrary<T>[] arbitraries;
 
 	@SuppressWarnings("unchecked")
-	public DefaultListCombinator(List<Arbitrary<T>> listOfArbitraries) {
+	public DefaultListCombinator(List<? extends Arbitrary<T>> listOfArbitraries) {
 		this(listOfArbitraries.toArray(new Arbitrary[0]));
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
index e5281d736..2c8b50ca7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
@@ -97,7 +97,7 @@ public static <T> Optional<ExhaustiveGenerator<List<T>>> shuffle(List<T> values,
 
 	public static <U, T> Optional<ExhaustiveGenerator<U>> flatMap(
 		ExhaustiveGenerator<T> base,
-		Function<T, Arbitrary<U>> mapper,
+		Function<? super T, ? extends Arbitrary<U>> mapper,
 		long maxNumberOfSamples
 	) {
 		Optional<Long> optionalMaxCount = FlatMappedExhaustiveGenerator.calculateMaxCounts(base, mapper, maxNumberOfSamples);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
index 8241a4ada..3db50eaa9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
@@ -10,11 +10,11 @@
 public class FlatMappedExhaustiveGenerator<U, T> implements ExhaustiveGenerator<U> {
 	private final ExhaustiveGenerator<T> baseGenerator;
 	private final long maxCount;
-	private final Function<T, Arbitrary<U>> mapper;
+	private final Function<? super T, ? extends Arbitrary<U>> mapper;
 
 	public static <T, U> Optional<Long> calculateMaxCounts(
 		ExhaustiveGenerator<T> baseGenerator,
-		Function<T, Arbitrary<U>> mapper,
+		Function<? super T, ? extends Arbitrary<U>> mapper,
 		long maxNumberOfSamples
 	) {
 		long choices = 0;
@@ -31,7 +31,7 @@ public static <T, U> Optional<Long> calculateMaxCounts(
 		return Optional.of(choices);
 	}
 
-	public FlatMappedExhaustiveGenerator(ExhaustiveGenerator<T> baseGenerator, long maxCount, Function<T, Arbitrary<U>> mapper) {
+	public FlatMappedExhaustiveGenerator(ExhaustiveGenerator<T> baseGenerator, long maxCount, Function<? super T, ? extends Arbitrary<U>> mapper) {
 		this.baseGenerator = baseGenerator;
 		this.maxCount = maxCount;
 		this.mapper = mapper;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java
index 67fcb862d..9edab461f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java
@@ -13,12 +13,12 @@
 abstract class AbstractFunctionGenerator<F, R> implements RandomGenerator<F> {
 	final Class<F> functionalType;
 	final RandomGenerator<R> resultGenerator;
-	final List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions;
+	final List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions;
 
 	AbstractFunctionGenerator(
 		Class<F> functionalType,
 		RandomGenerator<R> resultGenerator,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions
 	) {
 		this.functionalType = functionalType;
 		this.resultGenerator = resultGenerator;
@@ -70,7 +70,7 @@ private Object handleToStringOfConstantMethod(final R constant) {
 	// Returns result wrapped in array to allow null as result
 	protected Optional<Object[]> conditionalResult(Object[] args) {
 		Optional<Object[]> conditionalResult = Optional.empty();
-		for (Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> condition : conditions) {
+		for (Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> condition : conditions) {
 			List<Object> params = Arrays.asList(args);
 			if (condition.get1().test(params)) {
 				Object[] result = new Object[]{condition.get2().apply(params)};
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java
index 31743d2b8..4343f8304 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java
@@ -11,7 +11,7 @@ public class ConstantFunctionGenerator<F, R> extends AbstractFunctionGenerator<F
 	public ConstantFunctionGenerator(
 		Class<F> functionalType,
 		RandomGenerator<R> resultGenerator,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions
 	) {
 		super(functionalType, resultGenerator, conditions);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
index 6f0ccd185..cedb08da0 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
@@ -8,7 +8,7 @@
 
 class FrequencyGenerator<T> extends ChooseRandomlyByFrequency<T> implements RandomGenerator<T> {
 
-	FrequencyGenerator(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	FrequencyGenerator(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		super(frequencies);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java
index f1750e2f5..12022209d 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java
@@ -18,7 +18,7 @@ public class FunctionGenerator<F, R> extends AbstractFunctionGenerator<F, R> {
 	public FunctionGenerator(
 		Class<F> functionalType,
 		RandomGenerator<R> resultGenerator,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions
 	) {
 		super(functionalType, resultGenerator, conditions);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
index cf1a30fd2..5e6fb3a1e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
@@ -174,12 +174,12 @@ public static <T> RandomGenerator<T> samples(T[] samples) {
 		return samplesFromShrinkables(shrinkables);
 	}
 
-	public static <T> RandomGenerator<T> frequency(List<Tuple2<Integer, T>> frequencies) {
+	public static <T> RandomGenerator<T> frequency(List<? extends Tuple2<Integer, ? extends T>> frequencies) {
 		return new FrequencyGenerator<>(frequencies);
 	}
 
 	public static <T> RandomGenerator<T> frequencyOf(
-			List<Tuple2<Integer, Arbitrary<T>>> frequencies,
+			List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies,
 			int genSize,
 			boolean withEmbeddedEdgeCases
 	) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
index 96ee7c9ca..c3bf061a7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
@@ -7,9 +7,9 @@
 
 public class ChooseValueShrinkable<T> extends AbstractValueShrinkable<T> {
 
-	private final List<T> values;
+	private final List<? extends T> values;
 
-	public ChooseValueShrinkable(T value, List<T> values) {
+	public ChooseValueShrinkable(T value, List<? extends T> values) {
 		super(value);
 		this.values = values;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java b/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
index 891abaf78..1b1faf86e 100644
--- a/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
+++ b/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
@@ -11,7 +11,7 @@ public class ChooseRandomlyByFrequency<T> implements Function<Random, T> {
 	private int size = 0;
 	private List<T> valuesToChooseFrom;
 
-	public ChooseRandomlyByFrequency(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	public ChooseRandomlyByFrequency(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		calculateUpperBorders(frequencies);
 		if (size <= 0) {
 			throw new JqwikException(String.format(
@@ -25,11 +25,11 @@ protected List<T> possibleValues() {
 		return valuesToChooseFrom;
 	}
 
-	private void calculateUpperBorders(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	private void calculateUpperBorders(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		List<T> values = new ArrayList<>(frequencies.size());
 		// Zero-frequency elements are possible, so we start with a list, and transform to array later
 		List<Integer> upperBounds = new ArrayList<>(frequencies.size());
-		for (Tuple.Tuple2<Integer, T> tuple : frequencies) {
+		for (Tuple.Tuple2<Integer, ? extends T> tuple : frequencies) {
 			int frequency = tuple.get1();
 			if (frequency <= 0)
 				continue;
diff --git a/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java b/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java
index fede04bb1..1e5cf24f7 100644
--- a/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java
+++ b/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java
@@ -240,7 +240,7 @@ void checkCountForQuery(@ForAll @IntRange(min = 1, max = 100) int anInt) {
 			Statistics.collect(anInt);
 
 			Statistics.coverage(coverage -> {
-				Predicate<List<Integer>> query = params -> params.get(0) <= 10;
+				Predicate<List<?>> query = params -> (Integer) params.get(0) <= 10;
 				coverage.checkQuery(query).count(c -> c == 10);
 			});
 		}
@@ -250,7 +250,7 @@ void checkPercentageForQuery(@ForAll @IntRange(min = 1, max = 100) int anInt) {
 			Statistics.collect(anInt);
 
 			Statistics.coverage(coverage -> {
-				Predicate<List<Integer>> query = params -> params.get(0) <= 50;
+				Predicate<List<?>> query = params -> (Integer) params.get(0) <= 50;
 				coverage.checkQuery(query).percentage(p -> {
 					assertThat(p).isCloseTo(50.0, Offset.offset(0.1));
 				});
@@ -263,7 +263,7 @@ void checkCountsAreAddedUp(@ForAll @IntRange(min = 1, max = 100) int anInt) {
 			Statistics.label("twice").collect(anInt);
 
 			Statistics.label("twice").coverage(coverage -> {
-				Predicate<List<Integer>> query = params -> params.get(0) <= 10;
+				Predicate<List<?>> query = params -> (Integer) params.get(0) <= 10;
 				coverage.checkQuery(query).count(c -> c == 20);
 			});
 		}
@@ -274,7 +274,7 @@ void queryWithWrongTypeFails(@ForAll int anInt) {
 			Statistics.collect(anInt);
 
 			Statistics.coverage(coverage -> {
-				Predicate<List<String>> query = params -> !params.get(0).isEmpty();
+				Predicate<List<?>> query = params -> !((String)params.get(0)).isEmpty();
 				coverage.checkQuery(query).count(c -> c == 10);
 			});
 		}
diff --git a/kotlin/build.gradle b/kotlin/build.gradle
index 311c9d46a..1b849e9f8 100644
--- a/kotlin/build.gradle
+++ b/kotlin/build.gradle
@@ -78,6 +78,7 @@ signing {
 tasks.withType(KotlinCompile).configureEach {
 	compilerOptions {
 		freeCompilerArgs = [
+			"-Xnullability-annotations=@org.jspecify.annotations:strict",
 			"-Xemit-jvm-type-annotations" // Required for annotations on type variables
 		]
 		apiVersion = KotlinVersion.KOTLIN_1_9
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
index a25baa473..b8a2771a8 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
@@ -9,10 +9,8 @@ import java.util.function.BiFunction
  * Convenience function for Kotlin to not use backticked `in` function.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <B, T> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
+fun <B, T: Any?> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
     where B : Any {
-    // This explicit conversion form combinator to toFunction is necessary
-    // since the implicit conversion (just handing in combinator) creates nullability warning
-    val toFunction: BiFunction<B, T, B> = BiFunction<B, T, B> { b, t -> combinator(b, t) }
+    val toFunction = BiFunction<B, T, B> { b, t -> combinator(b, t) }
     return this.use(arbitrary).`in`(toFunction)
 }
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
index 9a0b023e5..1b75d2170 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
@@ -15,7 +15,7 @@ import java.util.function.Function
  * of type [Sequence]
  */
 @API(status = EXPERIMENTAL, since = "1.6.0")
-class SequenceArbitrary<T>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<Sequence<T>>(),
+class SequenceArbitrary<T: Any?>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<Sequence<T>>(),
     SizableArbitrary<Sequence<T>> {
 
     private var listArbitrary: ListArbitrary<T>
@@ -80,7 +80,7 @@ class SequenceArbitrary<T>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<
      *
      * @return new arbitrary instance
      */
-    fun uniqueElements(by: Function<T, Any>): SequenceArbitrary<T> {
+    fun uniqueElements(by: Function<in T, *>): SequenceArbitrary<T> {
         val clone = typedClone<SequenceArbitrary<T>>()
         clone.listArbitrary = listArbitrary.uniqueElements(by)
         return clone
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt
index 67d802fd9..0b9ddd1c9 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt
@@ -19,4 +19,4 @@ fun <T> IteratorArbitrary<T>.ofSize(range: IntRange): IteratorArbitrary<T> = ofM
 fun <K, V> MapArbitrary<K, V>.ofSize(range: IntRange): MapArbitrary<K, V> = ofMinSize(range.first).ofMaxSize(range.last)
 
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <T, A> ArrayArbitrary<T, A>.ofSize(range: IntRange): ArrayArbitrary<T, A> = ofMinSize(range.first).ofMaxSize(range.last)
+fun <T, A: Any> ArrayArbitrary<T, A>.ofSize(range: IntRange): ArrayArbitrary<T, A> = ofMinSize(range.first).ofMaxSize(range.last)
diff --git a/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt b/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt
index 531372020..b0115584b 100644
--- a/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt
+++ b/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt
@@ -1,7 +1,7 @@
 package net.jqwik.kotlin
 
-import net.jqwik.api.Example
-import net.jqwik.api.ForAll
+import net.jqwik.api.*
+import net.jqwik.kotlin.api.any
 import net.jqwik.kotlin.api.anySubset
 import net.jqwik.kotlin.api.anyValue
 import net.jqwik.kotlin.api.ofSize
diff --git a/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt b/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt
index 62d84be0b..bed36587d 100644
--- a/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt
+++ b/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt
@@ -1,12 +1,10 @@
 package net.jqwik.kotlin
 
-import net.jqwik.api.ForAll
-import net.jqwik.api.Property
-import net.jqwik.api.PropertyDefaults
-import net.jqwik.api.Provide
+import net.jqwik.api.*
 import net.jqwik.api.lifecycle.AfterContainer
 import net.jqwik.api.lifecycle.AfterProperty
 import net.jqwik.kotlin.api.any
+import kotlin.random.Random
 
 @PropertyDefaults(tries = 10)
 internal class InternalModifierTests {