From b7b5d4bb222aba58d472b90ea7266aa988a20ec8 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Wed, 12 Oct 2022 10:41:24 +0800
Subject: [PATCH 1/2] Additional test coverage for `common_type_t`

Ensuring that common_type specializations are not doing evil.
---
 .../tests/P1522R1_difference_type/test.cpp    | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tests/std/tests/P1522R1_difference_type/test.cpp b/tests/std/tests/P1522R1_difference_type/test.cpp
index 3a0cb740e2..01ce691973 100644
--- a/tests/std/tests/P1522R1_difference_type/test.cpp
+++ b/tests/std/tests/P1522R1_difference_type/test.cpp
@@ -125,6 +125,18 @@ constexpr bool test_unsigned() {
     STATIC_ASSERT(SAME_AS<std::common_type_t<_Unsigned128, unsigned long long>, _Unsigned128>);
     STATIC_ASSERT(SAME_AS<std::common_type_t<_Unsigned128, _Signed128>, _Unsigned128>);
 
+    struct ConversionSource {
+        constexpr operator _Unsigned128() const {
+            return _Unsigned128{};
+        }
+    };
+    STATIC_ASSERT(SAME_AS<std::common_type_t<_Unsigned128, ConversionSource>, _Unsigned128>);
+
+    struct ConversionTarget {
+        constexpr ConversionTarget(_Unsigned128) {}
+    };
+    STATIC_ASSERT(SAME_AS<std::common_type_t<_Unsigned128, ConversionTarget>, ConversionTarget>);
+
 #ifdef __cpp_lib_concepts // TRANSITION, GH-395
     STATIC_ASSERT(std::_Integer_class<_Unsigned128>);
     STATIC_ASSERT(std::_Integer_like<_Unsigned128>);
@@ -446,6 +458,18 @@ constexpr bool test_signed() {
     STATIC_ASSERT(SAME_AS<std::common_type_t<_Signed128, long long>, _Signed128>);
     STATIC_ASSERT(SAME_AS<std::common_type_t<_Signed128, unsigned long long>, _Signed128>);
 
+    struct ConversionSource {
+        constexpr operator _Signed128() const {
+            return _Signed128{};
+        }
+    };
+    STATIC_ASSERT(SAME_AS<std::common_type_t<_Signed128, ConversionSource>, _Signed128>);
+
+    struct ConversionTarget {
+        constexpr ConversionTarget(_Signed128) {}
+    };
+    STATIC_ASSERT(SAME_AS<std::common_type_t<_Signed128, ConversionTarget>, ConversionTarget>);
+
 #ifdef __cpp_lib_concepts // TRANSITION, GH-395
     STATIC_ASSERT(std::_Integer_class<_Signed128>);
     STATIC_ASSERT(std::_Integer_like<_Signed128>);

From 52b96676a574f8c28261ae44ba0f2442cec3d500 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Wed, 12 Oct 2022 10:44:36 +0800
Subject: [PATCH 2/2] Remove unnecessary `common_type` specializations

And evil ones!
---
 stl/inc/__msvc_int128.hpp | 32 --------------------------------
 1 file changed, 32 deletions(-)

diff --git a/stl/inc/__msvc_int128.hpp b/stl/inc/__msvc_int128.hpp
index 8740686ae2..c1abd2979a 100644
--- a/stl/inc/__msvc_int128.hpp
+++ b/stl/inc/__msvc_int128.hpp
@@ -1026,22 +1026,6 @@ class numeric_limits<_Unsigned128> : public _Num_int_base {
     static constexpr int digits10   = 38;
 };
 
-#ifdef __cpp_lib_concepts
-template <integral _Ty>
-struct common_type<_Ty, _Unsigned128> {
-    using type = _Unsigned128;
-};
-template <integral _Ty>
-struct common_type<_Unsigned128, _Ty> {
-    using type = _Unsigned128;
-};
-#else // ^^^ defined(__cpp_lib_concepts) / !defined(__cpp_lib_concepts) vvv
-template <class _Ty>
-struct common_type<_Ty, _Unsigned128> : enable_if<is_integral_v<_Ty>, _Unsigned128> {};
-template <class _Ty>
-struct common_type<_Unsigned128, _Ty> : enable_if<is_integral_v<_Ty>, _Unsigned128> {};
-#endif // ^^^ !defined(__cpp_lib_concepts) ^^^
-
 struct _Signed128 : _Base128 {
     using _Signed_type   = _Signed128;
     using _Unsigned_type = _Unsigned128;
@@ -1427,22 +1411,6 @@ class numeric_limits<_Signed128> : public _Num_int_base {
     static constexpr int digits10 = 38;
 };
 
-#ifdef __cpp_lib_concepts
-template <integral _Ty>
-struct common_type<_Ty, _Signed128> {
-    using type = _Signed128;
-};
-template <integral _Ty>
-struct common_type<_Signed128, _Ty> {
-    using type = _Signed128;
-};
-#else // ^^^ defined(__cpp_lib_concepts) / !defined(__cpp_lib_concepts) vvv
-template <class _Ty>
-struct common_type<_Ty, _Signed128> : enable_if<is_integral_v<_Ty>, _Signed128> {};
-template <class _Ty>
-struct common_type<_Signed128, _Ty> : enable_if<is_integral_v<_Ty>, _Signed128> {};
-#endif // ^^^ !defined(__cpp_lib_concepts) ^^^
-
 template <>
 struct common_type<_Signed128, _Unsigned128> {
     using type = _Unsigned128;