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From: Jonathan Wakely <redi@gcc.gnu.org>
To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org
Subject: [gcc r10-10308] libstdc++: Constrain std::make_any [PR102894]
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https://gcc.gnu.org/g:923637b6cb70986e83ae0109ec4bcd26fdfe3624

commit r10-10308-g923637b6cb70986e83ae0109ec4bcd26fdfe3624
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Oct 22 22:55:00 2021 +0100

    libstdc++: Constrain std::make_any [PR102894]
    
    std::make_any should be constrained so it can only be called if the
    construction of the return value would be valid.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/102894
            * include/std/any (make_any): Add SFINAE constraint.
            * testsuite/20_util/any/102894.cc: New test.
    
    (cherry picked from commit 0c1f737a485f05c591c94b50acfb416c45a4c916)

Diff:
---
 libstdc++-v3/include/std/any                 | 13 +++++++++----
 libstdc++-v3/testsuite/20_util/any/102894.cc | 20 ++++++++++++++++++++
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/any b/libstdc++-v3/include/std/any
index 16723c3a304..1428858d600 100644
--- a/libstdc++-v3/include/std/any
+++ b/libstdc++-v3/include/std/any
@@ -411,16 +411,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// Exchange the states of two @c any objects.
   inline void swap(any& __x, any& __y) noexcept { __x.swap(__y); }
 
-  /// Create an any holding a @c _Tp constructed from @c __args.
+  /// Create an `any` holding a `_Tp` constructed from `__args...`.
   template <typename _Tp, typename... _Args>
-    any make_any(_Args&&... __args)
+    inline
+    enable_if_t<is_constructible_v<any, in_place_type_t<_Tp>, _Args...>, any>
+    make_any(_Args&&... __args)
     {
       return any(in_place_type<_Tp>, std::forward<_Args>(__args)...);
     }
 
-  /// Create an any holding a @c _Tp constructed from @c __il and @c __args.
+  /// Create an `any` holding a `_Tp` constructed from `__il` and `__args...`.
   template <typename _Tp, typename _Up, typename... _Args>
-    any make_any(initializer_list<_Up> __il, _Args&&... __args)
+    inline
+    enable_if_t<is_constructible_v<any, in_place_type_t<_Tp>,
+				   initializer_list<_Up>&, _Args...>, any>
+    make_any(initializer_list<_Up> __il, _Args&&... __args)
     {
       return any(in_place_type<_Tp>, __il, std::forward<_Args>(__args)...);
     }
diff --git a/libstdc++-v3/testsuite/20_util/any/102894.cc b/libstdc++-v3/testsuite/20_util/any/102894.cc
new file mode 100644
index 00000000000..66ea9a03fea
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/any/102894.cc
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++17 } }
+#include <any>
+
+template<typename T, typename = void>
+struct can_make_any
+: std::false_type
+{ };
+
+template<typename T>
+struct can_make_any<T, std::void_t<decltype(std::make_any<T>())>>
+: std::true_type
+{ };
+
+struct move_only
+{
+  move_only() = default;
+  move_only(move_only&&) = default;
+};
+
+static_assert( ! can_make_any<move_only>::value ); // PR libstdc++/102894