commit f2f48b68a6a586f40dd8ae0e6d391b7f88756eec
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Aug 28 23:41:13 2020

    libstdc++: Fix common_type specializations for duration
    
    My recent change to implement P0548 ("common_type and duration") was not
    correct. The result of common_type_t<duration<R,P>, duration<R,P>>
    should be duration<common_type_t<R>, P::type>, not duration<R, P::type>.
    The common_type specialization for two different duration types was
    correct, but the specializations for a single duration type (which only
    exist to optimize compilation time) were wrong.
    
    This fixes the partial specializations of common_type for a single
    duration type, and also the return types of duration::operator+ and
    duration::operator- which are supposed to use common_type_t<duration>.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/chrono (common_type): Fix partial specializations
            for a single duration type to use the common_type of the rep.
            (duration::operator+, duration::operator-): Fix return types
            to also use the common_type of the rep.
            * testsuite/20_util/duration/requirements/reduced_period.cc:
            Check duration using a rep that has common_type specialized.

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index fb251848da8..524d23ea6a7 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -114,13 +114,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Rep, typename _Period>
     struct common_type<chrono::duration<_Rep, _Period>,
 		       chrono::duration<_Rep, _Period>>
-    { using type = chrono::duration<_Rep, typename _Period::type>; };
+    {
+      using type = chrono::duration<typename common_type<_Rep>::type,
+				    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>; };
+    {
+      using type = chrono::duration<typename common_type<_Rep>::type,
+				    typename _Period::type>;
+    };
 
   // 20.11.4.3 specialization of common_type (for time_point, sfinae-friendly)
 
@@ -463,13 +469,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 	// 20.11.5.3 arithmetic
 
-	constexpr duration<rep, period>
+	constexpr duration<typename common_type<rep>::type, period>
 	operator+() const
-	{ return *this; }
+	{ return duration<typename common_type<rep>::type, period>(__r); }
 
-	constexpr duration<rep, period>
+	constexpr duration<typename common_type<rep>::type, period>
 	operator-() const
-	{ return duration(-__r); }
+	{ return duration<typename common_type<rep>::type, period>(-__r); }
 
 	_GLIBCXX17_CONSTEXPR duration&
 	operator++()
diff --git a/libstdc++-v3/testsuite/20_util/duration/requirements/reduced_period.cc b/libstdc++-v3/testsuite/20_util/duration/requirements/reduced_period.cc
index 9eb38a0e56f..4c7472699d2 100644
--- a/libstdc++-v3/testsuite/20_util/duration/requirements/reduced_period.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/requirements/reduced_period.cc
@@ -129,3 +129,55 @@ test02()
   static_assert( is_same<decltype(-d3), common_type<D3>::type>::value,
       "unary - returns the reduced duration" );
 }
+
+template<typename T>
+struct Number
+{
+  explicit
+  Number(T t = 0) : i(t)
+  { }
+
+  template<typename U, bool B = std::is_convertible<U, T>::value,
+	   typename = typename std::enable_if<B>::type>
+    explicit
+    Number(Number<U> n) : i(n.i)
+    { }
+
+  T i = 0;
+
+  Number& operator+=(Number n) { i += n.i; return *this; }
+  Number& operator-=(Number n) { i -= n.i; return *this; }
+  Number& operator*=(Number n) { i *= n.i; return *this; }
+  Number& operator/=(Number n) { i /= n.i; return *this; }
+  Number& operator%=(Number n) { i %= n.i; return *this; }
+
+  Number operator+(Number n) { return { i + n.i }; }
+  Number operator-(Number n) { return { i - n.i }; }
+  Number operator*(Number n) { return { i * n.i }; }
+  Number operator/(Number n) { return { i / n.i }; }
+  Number operator%(Number n) { return { i % n.i }; }
+};
+
+namespace std
+{
+  // Specialise common_type to give a different type
+  template<>
+    struct common_type<Number<int>, Number<int>>
+    { using type = Number<long>; };
+}
+
+void
+test03()
+{
+
+  using D4 = duration<Number<int>, ratio<49, 21>>;
+  static_assert( is_same<common_type<D4>::type,
+			 duration<Number<long>, ratio<7, 3>>>::value,
+      "common_type_t<duration<R,P>> uses common_type_t<R>" );
+
+  D4 d4;
+  static_assert( is_same<decltype(+d4), common_type<D4>::type>::value,
+      "unary + returns type with common_type_t<D4::rep> as rep" );
+  static_assert( is_same<decltype(-d4), common_type<D4>::type>::value,
+      "unary - returns type with common_type_t<D4::rep> as rep" );
+}