PR libstdc++/89102 implement new common_type rules (P0435R1, P0548R1)

This change ensures that std::common_type<> is a complete type (LWG
2408), and that std::common_type<T>, std::common_type<cv T1, cv T2>, and
std::common_type<T1, T2, R...> will use program-defined specializations
for std::common_type<T1, T2> (LWG 2465).

The implementation of common_type<T1, T2, R...> is changed to use
void_t, and the specializations for duration and time_point are modified
to also use void_t instead of depending on implementation details of
common_type.

	PR libstdc++/89102
	* doc/xml/manual/intro.xml: Document DR 2408 and 2465 changes.
	* include/std/chrono (__duration_common_type_wrapper): Replace with ...
	(__duration_common_type): New helper.
	(common_type<chrono::duration<R1, P2>, chrono::duration<R2, P2>>): Use
	__duration_common_type.
	(__timepoint_common_type_wrapper): Replace with ...
	(__timepoint_common_type): New helper.
	(common_type<chrono::time_point<C, D2>, chrono::time_point<C, D2>>):
	Use __time_point_common_type.
	* include/std/type_traits (common_type<>): Define, as per LWG 2408.
	(__common_type_impl): If either argument is transformed by decay,
	use the common_type of the decayed types.
	(__common_type_impl<_Tp, _Up, _Tp, _Up>): If the types are already
	decayed, use __do_common_type_impl to get the common_type.
	(common_type<_Tp>): Use common_type<_Tp, _Tp>.
	(__do_member_type_wrapper, __member_type_wrapper)
	(__expanded_common_type_wrapper): Remove.
	(__common_type_pack, __common_type_fold): New helpers.
	(common_type<_Tp, _Up, _Vp...>): Use new helpers instead of
	__member_type_wrapper and __expanded_common_type_wrapper.
	* testsuite/20_util/common_type/requirements/explicit_instantiation.cc:
	Test zero-length template argument list.
	* testsuite/20_util/common_type/requirements/sfinae_friendly_1.cc:
	Test single argument cases and argument types that should decay.
	* testsuite/20_util/common_type/requirements/sfinae_friendly_2.cc:
	Adjust expected error.
	* testsuite/20_util/duration/literals/range_neg.cc: Use zero for
	dg-error lineno.
	* testsuite/20_util/duration/requirements/typedefs_neg1.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg3.cc: Likewise.

From-SVN: r270987

2 files changed, 83 insertions, 64 deletions

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 1d6326c..b7c1d75 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -67,48 +67,51 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // 20.11.4.3 specialization of common_type (for duration, sfinae-friendly)
 
+  template<typename _CT, typename _Period1, typename _Period2, typename = void>
+    struct __duration_common_type
+    { };
+
   template<typename _CT, typename _Period1, typename _Period2>
-    struct __duration_common_type_wrapper
+    struct __duration_common_type<_CT, _Period1, _Period2,
+				  __void_t<typename _CT::type>>
     {
     private:
-      typedef __static_gcd<_Period1::num, _Period2::num> __gcd_num;
-      typedef __static_gcd<_Period1::den, _Period2::den> __gcd_den;
-      typedef typename _CT::type __cr;
-      typedef ratio<__gcd_num::value,
-        (_Period1::den / __gcd_den::value) * _Period2::den> __r;
+      using __gcd_num = __static_gcd<_Period1::num, _Period2::num>;
+      using __gcd_den = __static_gcd<_Period1::den, _Period2::den>;
+      using __cr = typename _CT::type;
+      using __r = ratio<__gcd_num::value,
+			(_Period1::den / __gcd_den::value) * _Period2::den>;
+
     public:
-      typedef __success_type<chrono::duration<__cr, __r>> type;
+      using type = chrono::duration<__cr, __r>;
     };
 
   template<typename _Period1, typename _Period2>
-    struct __duration_common_type_wrapper<__failure_type, _Period1, _Period2>
+    struct __duration_common_type<__failure_type, _Period1, _Period2>
     { typedef __failure_type type; };
 
   template<typename _Rep1, typename _Period1, typename _Rep2, typename _Period2>
     struct common_type<chrono::duration<_Rep1, _Period1>,
-             chrono::duration<_Rep2, _Period2>>
-    : public __duration_common_type_wrapper<typename __member_type_wrapper<
-             common_type<_Rep1, _Rep2>>::type, _Period1, _Period2>::type
+		       chrono::duration<_Rep2, _Period2>>
+    : __duration_common_type<common_type<_Rep1, _Rep2>, _Period1, _Period2>
     { };
 
   // 20.11.4.3 specialization of common_type (for time_point, sfinae-friendly)
 
+  template<typename _CT, typename _Clock, typename = void>
+    struct __timepoint_common_type
+    { };
+
   template<typename _CT, typename _Clock>
-    struct __timepoint_common_type_wrapper
+    struct __timepoint_common_type<_CT, _Clock, __void_t<typename _CT::type>>
     {
-      typedef __success_type<chrono::time_point<_Clock, typename _CT::type>>
-        type;
+      using type = chrono::time_point<_Clock, typename _CT::type>;
     };
 
-  template<typename _Clock>
-    struct __timepoint_common_type_wrapper<__failure_type, _Clock>
-    { typedef __failure_type type; };
-
   template<typename _Clock, typename _Duration1, typename _Duration2>
     struct common_type<chrono::time_point<_Clock, _Duration1>,
-             chrono::time_point<_Clock, _Duration2>>
-    : public __timepoint_common_type_wrapper<typename __member_type_wrapper<
-             common_type<_Duration1, _Duration2>>::type, _Clock>::type
+		       chrono::time_point<_Clock, _Duration2>>
+    : __timepoint_common_type<common_type<_Duration1, _Duration2>, _Clock>
     { };
 
   namespace chrono
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 1d14c75..ea733e7 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2106,6 +2106,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct conditional<false, _Iftrue, _Iffalse>
     { typedef _Iffalse type; };
 
+  // __void_t (std::void_t for C++11)
+  template<typename...> using __void_t = void;
+
   /// common_type
   template<typename... _Tp>
     struct common_type;
@@ -2115,65 +2118,81 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   struct __do_common_type_impl
   {
     template<typename _Tp, typename _Up>
-      static __success_type<typename decay<decltype
-			    (true ? std::declval<_Tp>()
-			     : std::declval<_Up>())>::type> _S_test(int);
+      using __cond_t
+	= decltype(true ? std::declval<_Tp>() : std::declval<_Up>());
+
+    template<typename _Tp, typename _Up>
+      static __success_type<typename decay<__cond_t<_Tp, _Up>>::type>
+      _S_test(int);
 
     template<typename, typename>
-      static __failure_type _S_test(...);
+      static __failure_type
+      _S_test(...);
   };
 
-  template<typename _Tp, typename _Up>
-    struct __common_type_impl
-    : private __do_common_type_impl
-    {
-      typedef decltype(_S_test<_Tp, _Up>(0)) type;
-    };
-
-  struct __do_member_type_wrapper
-  {
-    template<typename _Tp>
-      static __success_type<typename _Tp::type> _S_test(int);
+  // If sizeof...(T) is zero, there shall be no member type.
+  template<>
+    struct common_type<>
+    { };
 
-    template<typename>
-      static __failure_type _S_test(...);
-  };
+  // If sizeof...(T) is one, the same type, if any, as common_type_t<T0, T0>.
+  template<typename _Tp0>
+    struct common_type<_Tp0>
+    : public common_type<_Tp0, _Tp0>
+    { };
 
-  template<typename _Tp>
-    struct __member_type_wrapper
-    : private __do_member_type_wrapper
+  // If sizeof...(T) is two, ...
+  template<typename _Tp1, typename _Tp2,
+	   typename _Dp1 = typename decay<_Tp1>::type,
+	   typename _Dp2 = typename decay<_Tp2>::type>
+    struct __common_type_impl
     {
-      typedef decltype(_S_test<_Tp>(0)) type;
+      // If is_same_v<T1, D1> is false or is_same_v<T2, D2> is false,
+      // let C denote the same type, if any, as common_type_t<D1, D2>.
+      using type = common_type<_Dp1, _Dp2>;
     };
 
-  template<typename _CTp, typename... _Args>
-    struct __expanded_common_type_wrapper
+  template<typename _Tp1, typename _Tp2>
+    struct __common_type_impl<_Tp1, _Tp2, _Tp1, _Tp2>
+    : private __do_common_type_impl
     {
-      typedef common_type<typename _CTp::type, _Args...> type;
+      // Otherwise, if decay_t<decltype(false ? declval<D1>() : declval<D2>())>
+      // denotes a valid type, let C denote that type.
+      using type = decltype(_S_test<_Tp1, _Tp2>(0));
     };
 
-  template<typename... _Args>
-    struct __expanded_common_type_wrapper<__failure_type, _Args...>
-    { typedef __failure_type type; };
+  // If sizeof...(T) is two, ...
+  template<typename _Tp1, typename _Tp2>
+    struct common_type<_Tp1, _Tp2>
+    : public __common_type_impl<_Tp1, _Tp2>::type
+    { };
 
-  template<>
-    struct common_type<>
+  template<typename...>
+    struct __common_type_pack
     { };
 
-  template<typename _Tp>
-    struct common_type<_Tp>
-    : common_type<_Tp, _Tp>
+  template<typename, typename, typename = void>
+    struct __common_type_fold;
+
+  // If sizeof...(T) is greater than two, ...
+  template<typename _Tp1, typename _Tp2, typename... _Rp>
+    struct common_type<_Tp1, _Tp2, _Rp...>
+    : public __common_type_fold<common_type<_Tp1, _Tp2>,
+				__common_type_pack<_Rp...>>
     { };
 
-  template<typename _Tp, typename _Up>
-    struct common_type<_Tp, _Up>
-    : public __common_type_impl<_Tp, _Up>::type
+  // Let C denote the same type, if any, as common_type_t<T1, T2>.
+  // If there is such a type C, type shall denote the same type, if any,
+  // as common_type_t<C, R...>.
+  template<typename _CTp, typename... _Rp>
+    struct __common_type_fold<_CTp, __common_type_pack<_Rp...>,
+			      __void_t<typename _CTp::type>>
+    : public common_type<typename _CTp::type, _Rp...>
     { };
 
-  template<typename _Tp, typename _Up, typename... _Vp>
-    struct common_type<_Tp, _Up, _Vp...>
-    : public __expanded_common_type_wrapper<typename __member_type_wrapper<
-               common_type<_Tp, _Up>>::type, _Vp...>::type
+  // Otherwise, there shall be no member type.
+  template<typename _CTp, typename _Rp>
+    struct __common_type_fold<_CTp, _Rp, void>
     { };
 
   template<typename _Tp, bool = is_enum<_Tp>::value>
@@ -2446,9 +2465,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<bool _Cond, typename _Tp = void>
     using __enable_if_t = typename enable_if<_Cond, _Tp>::type;
 
-  // __void_t (std::void_t for C++11)
-  template<typename...> using __void_t = void;
-
 #if __cplusplus >= 201703L || !defined(__STRICT_ANSI__) // c++17 or gnu++11
 #define __cpp_lib_void_t 201411
   /// A metafunction that always yields void, used for detecting valid types.