libstdc++: Make std::chrono::duration use reduced ratio for period

This implements the changes from P0548 "common_type and duration". That
was a change for C++17, but as it corrects some issues introduced by DRs
I'm also treating it as a DR and changing it for all modes from C++11
up.

The main change is that duration<R,P>::period no longer denotes P, but
rather P::type, the reduced ratio. The unary operator+ and operator-
members of duration should now return a duration using that reduced
ratio.

The requirement that common_type<T>::type is the same type as
common_type<T, T>::type (rather than simply T) was already implemented
for PR 89102.

The standard says that duration::operator+() and duration::operator-()
should return common_type_t<duration>, but that seems unnecessarily
expensive to compute. This change just uses duration<rep, period> which
is the same type, so we don't need to instantiate common_type.

As an optimization, this also adds partial specializations of
common_type for two durations of the same type, a single duration, two
time_points of the same type, and a single time_point. These
specializations avoid instantiating other specializations of common_type
and one or both of __duration_common_type or __timepoint_common_type for
the cases where the answer is trivial to obtain.

libstdc++-v3/ChangeLog:

	* include/std/chrono (__duration_common_type): Ensure the
	reduced ratio is used. Remove unused partial specialization
	using __failure_type.
	(common_type): Pass reduced ratios to __duration_common_type.
	Add partial specializations for simple cases involving a single
	duration or time_point type.
	(duration::period): Use reduced ratio.
	(duration::operator+(), duration::operator-()): Return duration
	type using the reduced ratio.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc:
	Adjust expected errors.
	* testsuite/20_util/duration/requirements/reduced_period.cc: New test.

1 files changed, 35 insertions, 10 deletions

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 9fc8f56..fb25184 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -94,13 +94,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			(_Period1::den / __gcd_den::value) * _Period2::den>;
 
     public:
-      using type = chrono::duration<__cr, __r>;
+      using type = chrono::duration<__cr, typename __r::type>;
     };
 
-  template<typename _Period1, typename _Period2>
-    struct __duration_common_type<__failure_type, _Period1, _Period2>
-    { typedef __failure_type type; };
-
   /// @endcond
 
   /// Specialization of common_type for chrono::duration types.
@@ -108,9 +104,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Rep1, typename _Period1, typename _Rep2, typename _Period2>
     struct common_type<chrono::duration<_Rep1, _Period1>,
 		       chrono::duration<_Rep2, _Period2>>
-    : __duration_common_type<common_type<_Rep1, _Rep2>, _Period1, _Period2>
+    : __duration_common_type<common_type<_Rep1, _Rep2>,
+			     typename _Period1::type,
+			     typename _Period2::type>
     { };
 
+  /// Specialization of common_type for two identical chrono::duration types.
+  /// @relates duration
+  template<typename _Rep, typename _Period>
+    struct common_type<chrono::duration<_Rep, _Period>,
+		       chrono::duration<_Rep, _Period>>
+    { using type = chrono::duration<_Rep, typename _Period::type>; };
+
+  /// Specialization of common_type for one chrono::duration type.
+  /// @relates duration
+  template<typename _Rep, typename _Period>
+    struct common_type<chrono::duration<_Rep, _Period>>
+    { using type = chrono::duration<_Rep, typename _Period::type>; };
+
   // 20.11.4.3 specialization of common_type (for time_point, sfinae-friendly)
 
   /// @cond undocumented
@@ -135,6 +146,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : __timepoint_common_type<common_type<_Duration1, _Duration2>, _Clock>
     { };
 
+  /// Specialization of common_type for two identical chrono::time_point types.
+  /// @relates time_point
+  template<typename _Clock, typename _Duration>
+    struct common_type<chrono::time_point<_Clock, _Duration>,
+		       chrono::time_point<_Clock, _Duration>>
+    { using type = chrono::time_point<_Clock, _Duration>; };
+
+  /// Specialization of common_type for one chrono::time_point type.
+  /// @relates time_point
+  template<typename _Clock, typename _Duration>
+    struct common_type<chrono::time_point<_Clock, _Duration>>
+    { using type = chrono::time_point<_Clock, _Duration>; };
+
   // @} group chrono
 
   namespace chrono
@@ -401,8 +425,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       public:
 
-	typedef _Rep						rep;
-	typedef _Period 					period;
+	using rep = _Rep;
+	using period = typename _Period::type;
 
 	static_assert(!__is_duration<_Rep>::value, "rep cannot be a duration");
 	static_assert(__is_ratio<_Period>::value,
@@ -438,11 +462,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	{ return __r; }
 
 	// 20.11.5.3 arithmetic
-	constexpr duration
+
+	constexpr duration<rep, period>
 	operator+() const
 	{ return *this; }
 
-	constexpr duration
+	constexpr duration<rep, period>
 	operator-() const
 	{ return duration(-__r); }