Fix std::visit to support arbitrary callables

The __visitor_result_type helper didn't use std::invoke and so didn't
compile when the visitor was a pointer-to-member rather than a function
object. Use std::invoke_result instead.

	* include/std/variant (__variant_idx_cookie): Add member type.
	(__visitor_result_type): Remove.
	(__do_visit): Use invoke_result instead of __visitor_result_type.
	* testsuite/20_util/variant/visit.cc: New test.

From-SVN: r270237

libstdc++-v3/ChangeLog

@@ -1,5 +1,10 @@
 2019-04-09  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/variant (__variant_idx_cookie): Add member type.
+	(__visitor_result_type): Remove.
+	(__do_visit): Use invoke_result instead of __visitor_result_type.
+	* testsuite/20_util/variant/visit.cc: New test.
+
 	PR libstdc++/90008
 	* include/std/variant (_VARIANT_RELATION_FUNCTION_TEMPLATE): Remove
 	unused capture.

libstdc++-v3/include/std/variant

@@ -178,7 +178,7 @@ namespace __variant
   // used for raw visitation
   struct __variant_cookie {};
   // used for raw visitation with indices passed in
-  struct __variant_idx_cookie {};
+  struct __variant_idx_cookie { using type = __variant_idx_cookie; };
   // a more explanatory name than 'true'
   inline constexpr auto __visit_with_index = bool_constant<true>{};
 
@@ -1613,27 +1613,18 @@ namespace __variant
       return __detail::__variant::__get<_Np>(std::move(__v));
     }
 
-  template<bool __use_index, typename _Visitor, typename... _Variants>
-    decltype(auto)
-    __visitor_result_type(_Visitor&& __visitor, _Variants&&... __variants)
-    {
-      if constexpr(__use_index)
-        return __detail::__variant::__variant_idx_cookie{};
-      else
-	return std::forward<_Visitor>(__visitor)(
-	  std::get<0>(std::forward<_Variants>(__variants))...);
-    }
-
   template<bool __use_index,
 	   bool __same_return_types,
 	   typename _Visitor, typename... _Variants>
     constexpr decltype(auto)
     __do_visit(_Visitor&& __visitor, _Variants&&... __variants)
     {
-      using _Result_type =
-	decltype(__visitor_result_type<__use_index>(
-	           std::forward<_Visitor>(__visitor),
-	           std::forward<_Variants>(__variants)...));
+      using _Deduced_type = std::invoke_result<_Visitor,
+	decltype(std::get<0>(std::declval<_Variants>()))...>;
+
+      using _Result_type = typename std::conditional_t<__use_index,
+	__detail::__variant::__variant_idx_cookie,
+	_Deduced_type>::type;
 
       constexpr auto& __vtable = __detail::__variant::__gen_vtable<
 	__same_return_types,
@@ -1663,7 +1654,6 @@ namespace __variant
       if ((__variants.valueless_by_exception() || ...))
 	__throw_bad_variant_access("Unexpected index");
 
-
       if constexpr (std::is_void_v<_Res>)
         (void) __do_visit<false, false>(std::forward<_Visitor>(__visitor),
 					std::forward<_Variants>(__variants)...);

libstdc++-v3/testsuite/20_util/variant/visit.cc

@@ -0,0 +1,73 @@
+// 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++17" }
+// { dg-do run { target c++17 } }
+
+#include <variant>
+#include <functional>
+#include <testsuite_hooks.h>
+
+// N.B. there are more std::visit tests in ./compile.cc and ./run.cc
+
+void
+test01()
+{
+  // Verify that visitation uses INVOKE and supports arbitrary callables.
+
+  struct X
+  {
+    int sum(int i) const { return i + n; }
+    int product(int i) const { return i * n; }
+    int n;
+  };
+
+  std::variant<X, X*, std::reference_wrapper<X>> vobj{X{1}};
+  int res = std::visit(&X::n, vobj);
+  VERIFY( res == 1 );
+
+  std::variant<int, short> varg{2};
+  res = std::visit(&X::sum, vobj, varg);
+  VERIFY( res == 3 );
+
+  X x{4};
+  vobj = &x;
+  res = std::visit(&X::n, vobj);
+  VERIFY( res == 4 );
+
+  varg.emplace<short>(5);
+  res = std::visit(&X::sum, vobj, varg);
+  VERIFY( res == 9 );
+
+  x.n = 6;
+  res = std::visit(&X::product, vobj, varg);
+  VERIFY( res == 30 );
+
+  vobj = std::ref(x);
+  x.n = 7;
+  res = std::visit(&X::n, vobj);
+  VERIFY( res == 7 );
+
+  res = std::visit(&X::product, vobj, varg);
+  VERIFY( res == 35 );
+}
+
+int
+main()
+{
+  test01();
+}