Implement std::visit<R> for C++2a (P0655R1)

	* doc/xml/manual/status_cxx2020.xml: Update status.
	* include/std/variant (visit<R>): Define for C++2a (P0655R1).
	* testsuite/20_util/variant/visit_r.cc: New test.

From-SVN: r270176

libstdc++-v3/ChangeLog

 2019-04-05  Jonathan Wakely  <jwakely@redhat.com>
 
+	* doc/xml/manual/status_cxx2020.xml: Update status.
+	* include/std/variant (visit<R>): Define for C++2a (P0655R1).
+	* testsuite/20_util/variant/visit_r.cc: New test.
+
 	* include/bits/fs_dir.h (directory_iterator::operator*)
 	(directory_iterator::operator->): Add noexcept.
 	(operator==, operator!=): Replace namespace-scope equality operators

libstdc++-v3/doc/xml/manual/status_cxx2020.xml

@@ -712,14 +712,13 @@ Feature-testing recommendations for C++</link>.
     </row>
 
     <row>
-      <?dbhtml bgcolor="#C8B0B0" ?>
       <entry>  <code>visit&lt;R&gt;</code>: Explicit Return Type for <code>visit</code> </entry>
       <entry>
         <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0655r1.pdf">
 	P0655R1
 	</link>
       </entry>
-      <entry align="center"> </entry>
+      <entry align="center"> 9.1 </entry>
       <entry />
     </row>
 

libstdc++-v3/include/std/variant

@@ -1610,6 +1610,23 @@ namespace __variant
 			std::forward<_Variants>(__variants)...);
     }
 
+#if __cplusplus > 201703L
+  template<typename _Res, typename _Visitor, typename... _Variants>
+    constexpr _Res
+    visit(_Visitor&& __visitor, _Variants&&... __variants)
+    {
+      if ((__variants.valueless_by_exception() || ...))
+	__throw_bad_variant_access("Unexpected index");
+
+      if constexpr (std::is_void_v<_Res>)
+	(void) __do_visit(std::forward<_Visitor>(__visitor),
+			  std::forward<_Variants>(__variants)...);
+      else
+	return __do_visit(std::forward<_Visitor>(__visitor),
+			  std::forward<_Variants>(__variants)...);
+    }
+#endif
+
   template<bool, typename... _Types>
     struct __variant_hash_call_base_impl
     {

libstdc++-v3/testsuite/20_util/variant/visit_r.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 run { target c++2a } }
+
+#include <variant>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  struct Visitor
+  {
+    int operator()(int, void*) const { return 0; }
+    int operator()(char& c, void* p) const { return &c == p; }
+    int operator()(int i, const char* s) const { return s[i] == '\0'; }
+    int operator()(char c, const char* s) const { return c == *s; }
+  };
+
+  std::variant<int, char> v1{'c'};
+  std::variant<void*, const char*> v2{"chars"};
+
+  auto res = std::visit<bool>(Visitor{}, v1, v2);
+  static_assert(std::is_same_v<decltype(res), bool>);
+  VERIFY( res == true );
+
+  static_assert(std::is_void_v<decltype(std::visit<void>(Visitor{}, v1, v2))>);
+}
+
+int
+main()
+{
+  test01();
+}