diff --git a/fixed_lib/include/fixedmath/detail/common.h b/fixed_lib/include/fixedmath/detail/common.h
index e02d18a..d541799 100644
--- a/fixed_lib/include/fixedmath/detail/common.h
+++ b/fixed_lib/include/fixedmath/detail/common.h
@@ -1,25 +1,29 @@
 #pragma once
 
 #include "../types.h"
-#include <type_traits>
-
-#define FIXEDMATH_PUBLIC gnu::visibility("default")
+#include <fixedmath/detail/type_traits.h>
 
 namespace fixedmath::inline v2::detail
   {
 
 template<int digits>
 [[gnu::const, gnu::always_inline]]
-constexpr auto unsigned_shift_left_signed(fixed_internal value) noexcept -> fixed_internal
+constexpr auto unsigned_shift_left_signed(std::signed_integral auto value) noexcept -> fixed_internal
   {
-  using unsigned_internal = std::make_unsigned_t<fixed_internal>;
   return static_cast<fixed_internal>(
-    (static_cast<unsigned_internal>(value) << digits)
-    | (static_cast<unsigned_internal>(value) & (unsigned_internal(1) << 63u))
+    (static_cast<fixed_internal_unsigned>(value) << digits)
+    | (static_cast<fixed_internal_unsigned>(value) & (fixed_internal_unsigned(1) << 63u))
   );
   }
 
-template<typename value_type>
+template<int digits>
+[[nodiscard, gnu::const, gnu::always_inline]]
+constexpr auto unsigned_shift_left_unsigned(std::unsigned_integral auto value) noexcept -> fixed_internal
+  {
+  return static_cast<fixed_internal>(static_cast<fixed_internal_unsigned>(value) << digits);
+  }
+
+template<std::integral value_type>
 [[nodiscard, gnu::const, gnu::always_inline]]
 constexpr auto promote_type_to_signed(value_type value) noexcept
   {
@@ -32,17 +36,9 @@ constexpr auto promote_type_to_signed(value_type value) noexcept
     }
   }
 
-template<int digits>
-[[nodiscard, gnu::const, gnu::always_inline]]
-constexpr auto unsigned_shift_left_unsigned(fixed_internal value) noexcept -> fixed_internal
-  {
-  using unsigned_internal = std::make_unsigned_t<fixed_internal>;
-  return static_cast<fixed_internal>(static_cast<unsigned_internal>(value) << digits);
-  }
-
 [[nodiscard, gnu::const, gnu::always_inline]]
 ///\returns the highest power of 4 that is less than or equal to \ref value
-constexpr auto highest_pwr4_clz(fixed_internal_unsigned value) noexcept -> fixed_internal
+constexpr auto highest_pwr4_clz(concepts::internal_unsigned auto value) noexcept -> fixed_internal
   {
   if(value != 0) [[likely]]
     {
@@ -58,7 +54,7 @@ constexpr auto highest_pwr4_clz(fixed_internal_unsigned value) noexcept -> fixed
   }
 
 [[nodiscard, gnu::const, gnu::always_inline]]
-constexpr auto highest_pwr4(fixed_internal_unsigned value) noexcept -> fixed_internal
+constexpr auto highest_pwr4(std::unsigned_integral auto value) noexcept -> fixed_internal
   {
   // one starts at the highest power of four <= than the argument.
   fixed_internal_unsigned pwr4{1ll << 62};  // second-to-top bit set
@@ -70,27 +66,27 @@ constexpr auto highest_pwr4(fixed_internal_unsigned value) noexcept -> fixed_int
 
 template<int precision>
 [[nodiscard, gnu::const, gnu::always_inline]]
-constexpr auto mul_(fixed_internal x, fixed_internal y) noexcept -> fixed_internal
+constexpr auto mul_(concepts::internal auto x, concepts::internal auto y) noexcept -> fixed_internal
   {
   return (x * y) >> precision;
   }
 
 template<int precision>
 [[nodiscard, gnu::const, gnu::always_inline]]
-constexpr auto div_(fixed_internal x, fixed_internal y) noexcept -> fixed_internal
+constexpr auto div_(concepts::internal auto x, concepts::internal auto y) noexcept -> fixed_internal
   {
   return (x << precision) / y;
   }
 
 template<int precision>
 [[nodiscard, gnu::const, gnu::always_inline]]
-constexpr auto fix_(fixed_internal x) noexcept -> fixed_internal
+constexpr auto fix_(std::integral auto x) noexcept -> fixed_internal
   {
-  return (x << precision);
+  return fixed_internal(x) << precision;
   }
 
 [[nodiscard, gnu::const, gnu::always_inline]]
-constexpr auto set_sign(bool sign_, fixed_internal result) -> fixed_t
+constexpr auto set_sign(bool sign_, concepts::internal auto result) -> fixed_t
   {
   if(!sign_)
     return as_fixed(result);
@@ -104,5 +100,5 @@ constexpr void swap(T & a, T & b) noexcept
   a = b;
   b = temp;
   }
-  }  // namespace fixedmath::detail
+  }  // namespace fixedmath::inline v2::detail
 
diff --git a/fixed_lib/include/fixedmath/detail/type_traits.h b/fixed_lib/include/fixedmath/detail/type_traits.h
index 6c02b2c..ae44ba5 100644
--- a/fixed_lib/include/fixedmath/detail/type_traits.h
+++ b/fixed_lib/include/fixedmath/detail/type_traits.h
@@ -37,7 +37,6 @@ using promote_to_signed_t = signed_type_by_size_t<(sizeof(T) << 1)>;
 template<typename T>
 inline constexpr bool is_integral_v = std::is_integral_v<T>;
 
-
 template<typename T>
 inline constexpr bool is_fixed_point_v = std::is_same_v<T, fixed_t>;
 
@@ -70,6 +69,11 @@ inline constexpr bool one_of_is_double_v = std::is_same_v<T, double> || std::is_
 namespace fixedmath::inline v2::concepts
   {
 using std::integral;
+template<typename T>
+concept internal_unsigned = std::same_as<T, fixed_internal_unsigned>;
+
+template<typename T>
+concept internal = std::same_as<T, fixed_internal>;
 
 template<typename T>
 concept fixed_point = typetraits::is_fixed_point_v<T>;
@@ -91,6 +95,7 @@ concept arithmetic_and_one_is_fixed = typetraits::is_arithmetic_and_one_is_fixed
 
 template<typename T, typename U>
 concept one_of_is_double = typetraits::one_of_is_double_v<T, U>;
+
   }  // namespace fixedmath::inline v2::concepts
 
 namespace fixedmath::inline v2::detail
diff --git a/fixed_lib/include/fixedmath/math.h b/fixed_lib/include/fixedmath/math.h
index ce8a921..45eb3fd 100644
--- a/fixed_lib/include/fixedmath/math.h
+++ b/fixed_lib/include/fixedmath/math.h
@@ -226,7 +226,7 @@ struct subtract_t
   template<concepts::arithmetic supported_type1, concepts::arithmetic supported_type2>
     requires concepts::arithmetic_and_one_is_fixed<supported_type1, supported_type2>
   [[nodiscard, gnu::const, gnu::always_inline]]
-  constexpr auto operator()(supported_type1 lh, supported_type2 rh) noexcept
+  static constexpr auto operator()(supported_type1 lh, supported_type2 rh) noexcept
     {
     if constexpr(typetraits::one_of_is_double_v<supported_type1, supported_type2>)
       return detail::promote_to_double(lh) - detail::promote_to_double(rh);
diff --git a/unit_tests/CMakeLists.txt b/unit_tests/CMakeLists.txt
index 5f32678..12e4a4c 100644
--- a/unit_tests/CMakeLists.txt
+++ b/unit_tests/CMakeLists.txt
@@ -53,7 +53,7 @@ endfunction()
 add_fixedmath_ut( fixed_construction_ut )
 add_fixedmath_ut( type_convertions_ut )
 add_fixedmath_ut( addition_ut )
-
+add_fixedmath_ut( subtraction_ut )
 
 # add_compile_test( addition )
 # add_compile_test( substraction )
diff --git a/unit_tests/addition_ut.cc b/unit_tests/addition_ut.cc
index b35153c..40a5c02 100644
--- a/unit_tests/addition_ut.cc
+++ b/unit_tests/addition_ut.cc
@@ -42,6 +42,26 @@ int main()
       expect_neq(limits_::lowest() + 1, limits_::quiet_NaN());
 
       expect_neq(as_fixed(limits_::max().v - 65536) + 1_fix, limits_::quiet_NaN());
+      
+      expect( test_resulting_type<fixed_t>( int64_t(1) + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + int64_t(1) ) );
+      expect( test_resulting_type<fixed_t>( uint64_t(1) + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + uint64_t(1) ) );
+      expect( test_resulting_type<fixed_t>( 1_fix + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1 + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix + 1 ) );
+      expect( test_resulting_type<fixed_t>( int16_t(1) + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + int16_t(1) ) );
+      expect( test_resulting_type<fixed_t>( uint16_t(1) + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + uint16_t(1) ) );
+      expect( test_resulting_type<fixed_t>( int8_t(1) + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + int8_t(1) ) );
+      expect( test_resulting_type<fixed_t>( uint8_t(1) + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + uint8_t(1) ) );
+      expect( test_resulting_type<fixed_t>( 1.f + 1_fix ) );
+      expect( test_resulting_type<fixed_t>( 1_fix  + 1.f ) );
+      expect( test_resulting_type<double>( 1. + 1_fix ) );
+      expect( test_resulting_type<double>( 1_fix  + 1. ) );
       return {};
     };
     result |= run_constexpr_test(fn_tmpl);
diff --git a/unit_tests/subtraction_ut.cc b/unit_tests/subtraction_ut.cc
new file mode 100644
index 0000000..b5c7e42
--- /dev/null
+++ b/unit_tests/subtraction_ut.cc
@@ -0,0 +1,61 @@
+#include <unit_test_core.h>
+#include <unit_test_common.h>
+
+using boost::ut::operator""_test;
+using namespace metatests;
+using namespace fixedmath;
+
+int main()
+  {
+  test_result result;
+  using F = fixedmath::fixed_internal;
+  "addition"_test = [&result]
+  {
+    auto fn_tmpl = [] -> metatests::test_result
+    {
+      expect_eq(0.2_fix - 1.2_fix, -1_fix);
+      expect_eq(3.2_fix - 1.2_fix, 2_fix);
+      expect_eq(-4.2_fix - 1.2_fix, -5.4_fix);
+      expect_eq(-4.2_fix + 1.2_fix, -3_fix);
+      
+      static constexpr auto approx{std::numeric_limits<double>::epsilon()};
+      expect_approx(1_fix - 1., 1. - 1., approx);
+      expect_approx(2_fix - 1., 2. - 1., approx);
+      expect_approx(-2_fix - 1., -2. - 1., approx);
+      expect_approx(-2_fix + 1., -2. + 1., approx);
+      
+      expect_eq(-10.5_fix - uint8_t(10), -20.5_fix);
+      expect_eq(-10.5_fix - uint16_t(10), -20.5_fix);
+      expect_eq(-10.5_fix - uint32_t(10), -20.5_fix);
+      expect_eq(-10.5_fix - uint64_t(10), -20.5_fix);
+      expect_eq(uint8_t(10) - 10.5_fix, -.5_fix);
+      expect_eq(uint16_t(10) - 10.5_fix, -.5_fix);
+      expect_eq(uint32_t(10) - 10.5_fix, -.5_fix);
+      expect_eq(uint64_t(10) - 10.5_fix, -.5_fix);
+      expect_neq(limits_::max() - 1, limits_::quiet_NaN());
+
+      expect(test_resulting_type<fixed_t>(int64_t(1) - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - int64_t(1)));
+      expect(test_resulting_type<fixed_t>(uint64_t(1) - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - uint64_t(1)));
+      expect(test_resulting_type<fixed_t>(1_fix - 1_fix));
+      expect(test_resulting_type<fixed_t>(1 - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - 1));
+      expect(test_resulting_type<fixed_t>(int16_t(1) - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - int16_t(1)));
+      expect(test_resulting_type<fixed_t>(uint16_t(1) - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - uint16_t(1)));
+      expect(test_resulting_type<fixed_t>(int8_t(1) - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - int8_t(1)));
+      expect(test_resulting_type<fixed_t>(uint8_t(1) - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - uint8_t(1)));
+      expect(test_resulting_type<fixed_t>(1.f - 1_fix));
+      expect(test_resulting_type<fixed_t>(1_fix - 1.f));
+      expect(test_resulting_type<double>(1. - 1_fix));
+      expect(test_resulting_type<double>(1_fix - 1.));
+      return {};
+    };
+    result |= run_constexpr_test(fn_tmpl);
+    result |= run_consteval_test(fn_tmpl);
+  };
+  }