Enhanced type narrowing logic for "x == <literal>" type guard pattern…

… to handle the case where `x` is a type variable with a literal upper bound or value constraints that are literals. This addresses #9300.

packages/pyright-internal/src/analyzer/typeGuards.ts

@@ -2468,7 +2468,7 @@ function narrowTypeForLiteralComparison(
     isPositiveTest: boolean,
     isIsOperator: boolean
 ): Type {
-    return mapSubtypes(referenceType, (subtype) => {
+    return evaluator.mapSubtypesExpandTypeVars(referenceType, /* options */ undefined, (subtype) => {
         subtype = evaluator.makeTopLevelTypeVarsConcrete(subtype);
 
         if (isAnyOrUnknown(subtype)) {

packages/pyright-internal/src/tests/samples/typeNarrowingLiteral1.py

@@ -1,7 +1,7 @@
 # This sample tests the type analyzer's type narrowing
 # logic for literals.
 
-from typing import Literal, Union
+from typing import Literal, TypeVar, Union
 
 
 def func1(p1: Literal["a", "b", "c"]):
@@ -28,3 +28,27 @@ def func2(p1: Literal[1, 4, 7]):
 def func3(a: Union[int, None]):
     if a == 1 or a == 2:
         reveal_type(a, expected_text="Literal[1, 2]")
+
+
+T = TypeVar("T", bound=Literal["a", "b"])
+
+
+def func4(x: T) -> T:
+    if x == "a":
+        reveal_type(x, expected_text="Literal['a']")
+        return x
+    else:
+        reveal_type(x, expected_text="Literal['b']")
+        return x
+
+
+S = TypeVar("S", Literal["a"], Literal["b"])
+
+
+def func5(x: S) -> S:
+    if x == "a":
+        reveal_type(x, expected_text="Literal['a']")
+        return x
+    else:
+        reveal_type(x, expected_text="Literal['b']")
+        return x