Make std::invoke usable in constant expressions

	* include/std/functional (invoke): Add constexpr for C++20.
	* include/std/version (__cpp_lib_constexpr_invoke): Define.
	* testsuite/20_util/function_objects/invoke/constexpr.cc: New test.

From-SVN: r277343

libstdc++-v3/ChangeLog

 2019-10-23  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/functional (invoke): Add constexpr for C++20.
+	* include/std/version (__cpp_lib_constexpr_invoke): Define.
+	* testsuite/20_util/function_objects/invoke/constexpr.cc: New test.
+
 	PR c++/91369 Implement P0784R7 changes to allocation and construction
 	* include/bits/alloc_traits.h: Include <bits/stl_construct.h>.
 	(allocator_traits::_S_allocate, allocator_traits::_S_construct)

libstdc++-v3/include/std/functional

@@ -72,19 +72,22 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
-#if __cplusplus > 201402L
-# define __cpp_lib_invoke 201411
+#if __cplusplus >= 201703L
+# define __cpp_lib_invoke 201411L
+# if __cplusplus > 201703L
+#  define __cpp_lib_constexpr_invoke 201907L
+# endif
 
   /// Invoke a callable object.
   template<typename _Callable, typename... _Args>
-    inline invoke_result_t<_Callable, _Args...>
+    inline _GLIBCXX20_CONSTEXPR invoke_result_t<_Callable, _Args...>
     invoke(_Callable&& __fn, _Args&&... __args)
     noexcept(is_nothrow_invocable_v<_Callable, _Args...>)
     {
       return std::__invoke(std::forward<_Callable>(__fn),
 			   std::forward<_Args>(__args)...);
     }
-#endif
+#endif // C++17
 
   template<typename _MemFunPtr,
 	   bool __is_mem_fn = is_member_function_pointer<_MemFunPtr>::value>

libstdc++-v3/include/std/version

@@ -158,6 +158,7 @@
 #endif
 #define __cpp_lib_constexpr 201711L
 #define __cpp_lib_constexpr_algorithms 201806L
+#define __cpp_lib_constexpr_invoke 201907L
 #if __cpp_impl_destroying_delete
 # define __cpp_lib_destroying_delete 201806L
 #endif

libstdc++-v3/testsuite/20_util/function_objects/invoke/constexpr.cc

+// Copyright (C) 2019 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <functional>
+
+#ifndef __cpp_lib_constexpr_invoke
+# error "Feature test macro for constexpr invoke is missing"
+#elif __cpp_lib_constexpr_invoke < 201907L
+# error "Feature test macro for constexpr invoke has wrong value"
+#endif
+
+constexpr int inc(int i) { return i + 1; }
+constexpr auto inc_f = &inc;
+static_assert( std::invoke(inc_f, 2) == 3);
+
+struct Dec
+{
+  constexpr int operator()(int i) const { return i - 1; }
+};
+
+static_assert( std::invoke(Dec{}, 5) == 4 );