libstdc++: Constrain chrono::duration conversions [LWG 2094]

The chrono::duration constructor that converts from another duration
type is meant to be constrained so that it doesn't participate in
overload resolution if the ratio of the periods cannot be represented as
a std::ratio.

Because our std::ratio_divide is not SFINAE-friendly the evaluation of
__is_harmonic results in an error outside the immediate context when an
overflow occurs. I intend to make ratio_divide (and ratio_multiply)
SFINAE-friendly in a future patch, but for now this patch just
introduces a new SFINAE-friendly alias template for the division.

The standard doesn't require it, but it also seems right to constrain
the constructor with std::is_convertible_v<_Rep2, rep>.

libstdc++-v3/ChangeLog:

	* include/std/chrono (duration::_S_gcd(intmax_t, intmax_t)):
	New helper function for finding GCD of two positive intmax_t
	values.
	(duration::__divide): New helper alias for dividing one period
	by another.
	(duration::__is_harmonic): Use __divide not ratio_divide.
	(duration(const duration<R2, P2>&)): Require the duration rep
	types to be convertible.
	* testsuite/20_util/duration/cons/dr2094.cc: New test.
	* testsuite/20_util/duration/requirements/reduced_period.cc:
	Fix definition of unused member functions in test type.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc:
	Adjust expected errors.

libstdc++-v3/include/std/chrono

@@ -424,10 +424,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	template<typename _Rep2>
 	  using __is_float = treat_as_floating_point<_Rep2>;
 
+	static constexpr intmax_t
+	_S_gcd(intmax_t __m, intmax_t __n) noexcept
+	{
+	  // Duration only allows positive periods so we don't need to
+	  // support negative values here (unlike __static_gcd and std::gcd).
+	  return (__m == 0) ? __n : (__n == 0) ? __m : _S_gcd(__n, __m % __n);
+	}
+
+	// _GLIBCXX_RESOLVE_LIB_DEFECTS
+	// 2094. overflow shouldn't participate in overload resolution
+	// 3090. What is [2094] intended to mean?
+	// This only produces a valid type if no overflow occurs.
+	template<typename _R1, typename _R2,
+		 intmax_t __gcd1 = _S_gcd(_R1::num, _R2::num),
+		 intmax_t __gcd2 = _S_gcd(_R1::den, _R2::den)>
+	  using __divide = ratio<(_R1::num / __gcd1) * (_R2::den / __gcd2),
+				 (_R1::den / __gcd2) * (_R2::num / __gcd1)>;
+
 	// _Period2 is an exact multiple of _Period
 	template<typename _Period2>
 	  using __is_harmonic
-	    = __bool_constant<ratio_divide<_Period2, _Period>::den == 1>;
+	    = __bool_constant<__divide<_Period2, _Period>::den == 1>;
 
       public:
 
@@ -453,6 +471,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  : __r(static_cast<rep>(__rep)) { }
 
 	template<typename _Rep2, typename _Period2, typename = _Require<
+		 is_convertible<const _Rep2&, rep>,
 		 __or_<__is_float<rep>,
 		       __and_<__is_harmonic<_Period2>,
 			      __not_<__is_float<_Rep2>>>>>>

libstdc++-v3/testsuite/20_util/duration/cons/dr2094.cc

@@ -0,0 +1,64 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <chrono>
+
+void
+test01()
+{
+  using namespace std::chrono;
+  using std::exa;
+
+  // LWG 2094
+  // duration conversion overflow shouldn't participate in overload resolution
+  bool f(milliseconds);
+  void f(seconds);
+  duration<int,exa> r(1);
+  f(r);
+}
+
+void
+test02()
+{
+  struct Number
+  {
+    explicit
+    Number(int t = 0) : i(t)
+    { }
+
+    int 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 Number{ i + n.i }; }
+    Number operator-(Number n) { return Number{ i - n.i }; }
+    Number operator*(Number n) { return Number{ i * n.i }; }
+    Number operator/(Number n) { return Number{ i / n.i }; }
+    Number operator%(Number n) { return Number{ i % n.i }; }
+  };
+
+  using std::chrono::duration;
+
+  static_assert( ! std::is_constructible<duration<int>, duration<Number>>(),
+      "duration(const duration<R2, P2>&) constrained on R2 -> R conversion" );
+}

libstdc++-v3/testsuite/20_util/duration/requirements/reduced_period.cc

@@ -151,11 +151,11 @@ struct Number
   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 }; }
+  Number operator+(Number n) { return Number{ i + n.i }; }
+  Number operator-(Number n) { return Number{ i - n.i }; }
+  Number operator*(Number n) { return Number{ i * n.i }; }
+  Number operator/(Number n) { return Number{ i / n.i }; }
+  Number operator%(Number n) { return Number{ i % n.i }; }
 };
 
 namespace std

libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg2.cc

@@ -32,4 +32,5 @@ void test01()
 
 // { dg-error "must be a specialization of ratio" "" { target *-*-* } 0 }
 // { dg-prune-output "'num' is not a member of 'int'" }
+// { dg-prune-output "'den' is not a member of 'int'" }
 // { dg-prune-output "'int' is not a class, struct, or union type" }