[PATCH] Implement std::visit for C++2a (P0655R1)

[Jonathan Wakely]

        * 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.

Tested powerpc64le-linux, committed to trunk.


commit 60c01065cfdda4f93bd140b0ea37be16f537877c
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Apr 5 18:36:25 2019 +0100

    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.

diff --git a/libstdc++-v3/doc/xml/manual/status_cxx2020.xml 
b/libstdc++-v3/doc/xml/manual/status_cxx2020.xml
index d40185c5db6..cedb3d03066 100644
--- a/libstdc++-v3/doc/xml/manual/status_cxx2020.xml
+++ b/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>
 
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index e52aa403009..fdf04cf624a 100644
--- a/libstdc++-v3/include/std/variant
+++ b/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
     {
diff --git a/libstdc++-v3/testsuite/20_util/variant/visit_r.cc 
b/libstdc++-v3/testsuite/20_util/variant/visit_r.cc
new file mode 100644
index 00000000000..5eed0cf1a49
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/visit_r.cc
@@ -0,0 +1,49 @@
+// 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();
+}