[gcc r11-1858] libstdc++: Constrain std::make_optional

[Jonathan Wakely]

https://gcc.gnu.org/g:bcfe4681f9be68f96f0610f30356510ff518806b

commit r11-1858-gbcfe4681f9be68f96f0610f30356510ff518806b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jul 6 21:54:12 2020 +0100

    libstdc++: Constrain std::make_optional
    
    The standard rquires that std::make_optional is constrained similarly to
    the std::optional constructors, which our implementation fails to do.
    
    As a conforming extension this also adds a noexcept-specifier to each
    std::make_optional overload.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/optional (make_optional): Add enable_if
            constraints and noexcept-specifier to each overload.
            * testsuite/20_util/optional/make_optional-2.cc: New test.

Diff:
---
 libstdc++-v3/include/std/optional                  | 25 ++++--
 .../testsuite/20_util/optional/make_optional-2.cc  | 94 ++++++++++++++++++++++
 2 files changed, 111 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 785c434412d..24821f81d0f 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1220,19 +1220,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     swap(optional<_Tp>&, optional<_Tp>&) = delete;
 
   template<typename _Tp>
-    constexpr optional<decay_t<_Tp>>
+    constexpr
+    enable_if_t<is_constructible_v<decay_t<_Tp>, _Tp>,
+		optional<decay_t<_Tp>>>
     make_optional(_Tp&& __t)
-    { return optional<decay_t<_Tp>> { std::forward<_Tp>(__t) }; }
+    noexcept(is_nothrow_constructible_v<optional<decay_t<_Tp>>, _Tp>)
+    { return optional<decay_t<_Tp>>{ std::forward<_Tp>(__t) }; }
 
-  template<typename _Tp, typename ..._Args>
-    constexpr optional<_Tp>
+  template<typename _Tp, typename... _Args>
+    constexpr
+    enable_if_t<is_constructible_v<_Tp, _Args...>,
+		optional<_Tp>>
     make_optional(_Args&&... __args)
-    { return optional<_Tp> { in_place, std::forward<_Args>(__args)... }; }
+    noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
+    { return optional<_Tp>{ in_place, std::forward<_Args>(__args)... }; }
 
-  template<typename _Tp, typename _Up, typename ..._Args>
-    constexpr optional<_Tp>
+  template<typename _Tp, typename _Up, typename... _Args>
+    constexpr
+    enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
+		optional<_Tp>>
     make_optional(initializer_list<_Up> __il, _Args&&... __args)
-    { return optional<_Tp> { in_place, __il, std::forward<_Args>(__args)... }; }
+    noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>)
+    { return optional<_Tp>{ in_place, __il, std::forward<_Args>(__args)... }; }
 
   // Hash.
 
diff --git a/libstdc++-v3/testsuite/20_util/optional/make_optional-2.cc b/libstdc++-v3/testsuite/20_util/optional/make_optional-2.cc
new file mode 100644
index 00000000000..65a1fc78d80
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/optional/make_optional-2.cc
@@ -0,0 +1,94 @@
+// { dg-do compile { target c++17 }  }
+
+// 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/>.
+
+#include <optional>
+
+int i;
+
+struct Cont
+{
+  Cont() noexcept;
+  Cont(Cont&&) noexcept;
+  Cont(const Cont&);
+  Cont(int);
+  Cont(std::initializer_list<int>, int) noexcept;
+  Cont(std::initializer_list<int>, const char*);
+};
+const Cont c{};
+
+template<typename T, typename = void>
+  struct can_make_optional1
+  : std::false_type
+  { };
+
+template<typename T>
+  struct can_make_optional1<T,
+      std::void_t<decltype(std::make_optional(std::declval<T>()))>>
+  : std::true_type
+  { };
+
+static_assert( can_make_optional1<int>::value );
+static_assert( noexcept(std::make_optional(1)) );
+static_assert( can_make_optional1<int&>::value );
+static_assert( noexcept(std::make_optional(i)) );
+static_assert( ! can_make_optional1<void>::value );
+static_assert( can_make_optional1<Cont>::value );
+static_assert( noexcept(std::make_optional(Cont{})) );
+static_assert( can_make_optional1<Cont>::value );
+static_assert( ! noexcept(std::make_optional(c)) );
+
+template<typename T, typename Arg, typename = void>
+  struct can_make_optional2
+  : std::false_type
+  { };
+
+template<typename T, typename Arg>
+  struct can_make_optional2<T, Arg,
+      std::void_t<decltype(std::make_optional<T>(std::declval<Arg>()))>>
+  : std::true_type
+  { };
+
+static_assert( can_make_optional2<int, int>::value );
+static_assert( noexcept(std::make_optional<int>(1)) );
+static_assert( can_make_optional2<int, int&>::value );
+static_assert( noexcept(std::make_optional(i)) );
+static_assert( ! can_make_optional2<void, void>::value );
+static_assert( can_make_optional2<Cont, Cont>::value );
+static_assert( noexcept(std::make_optional<Cont>({})) );
+static_assert( can_make_optional2<Cont, const Cont&>::value );
+static_assert( ! noexcept(std::make_optional(c)) );
+static_assert( can_make_optional2<Cont, int>::value );
+static_assert( ! noexcept(std::make_optional<Cont>(1)) );
+
+template<typename T, typename Arg, typename = void>
+  struct can_make_optional3
+  : std::false_type
+  { };
+
+template<typename T, typename Arg>
+  struct can_make_optional3<T, Arg,
+      std::void_t<decltype(std::make_optional<T>({1,2}, std::declval<Arg>()))>>
+  : std::true_type
+  { };
+
+static_assert( can_make_optional3<Cont, int>::value );
+static_assert( noexcept(std::make_optional<Cont>({1,2}, 1)) );
+static_assert( can_make_optional3<Cont, char*>::value );
+static_assert( ! noexcept(std::make_optional<Cont>({1,2}, "")) );
+static_assert( !can_make_optional3<Cont, int*>::value );