[committed] libstdc++: Do not use deduced return type for std::visit [PR 100384]

[Jonathan Wakely via Gcc-patches]

This avoids errors outside the immediate context when std::visit is an
overload candidate because of ADL, but not actually viable.

The solution is to give std::visit a non-deduced return type. New
helpers are introduced for that, and existing ones refactored slightly.

libstdc++-v3/ChangeLog:

        PR libstdc++/100384
        * include/std/variant (__get_t): New alias template yielding the
        return type of std::get<N> on a variant.
        (__visit_result_t): New alias template yielding the result of
        std::visit.
        (__same_types): Move into namespace __detail::__variant.
        (__check_visitor_results): Likewise. Use __invoke_result_t and
        __get_t.
        (__check_visitor_result): Remove.
        (visit): Use __visit_result_t for return type.
        * testsuite/20_util/variant/100384.cc: New test.

Tested powerpc64le-linux. Committed to trunk.

This should be backported too, as the std::visit on the branches has
this bug.


commit af5b2b911dd80ae9cc87404b7e7ab807cf6655d4
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue May 4 12:16:46 2021

    libstdc++: Do not use deduced return type for std::visit [PR 100384]
    
    This avoids errors outside the immediate context when std::visit is an
    overload candidate because of ADL, but not actually viable.
    
    The solution is to give std::visit a non-deduced return type. New
    helpers are introduced for that, and existing ones refactored slightly.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100384
            * include/std/variant (__get_t): New alias template yielding the
            return type of std::get<N> on a variant.
            (__visit_result_t): New alias template yielding the result of
            std::visit.
            (__same_types): Move into namespace __detail::__variant.
            (__check_visitor_results): Likewise. Use __invoke_result_t and
            __get_t.
            (__check_visitor_result): Remove.
            (visit): Use __visit_result_t for return type.
            * testsuite/20_util/variant/100384.cc: New test.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 1ef9b9725e2..9b2bc440841 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1063,6 +1063,25 @@ namespace __variant
                              std::index_sequence<__indices...>>
     : _Base_dedup<__indices, __poison_hash<remove_const_t<_Types>>>... { };
 
+  template<size_t _Np, typename _Variant>
+    using __get_t = decltype(std::get<_Np>(std::declval<_Variant>()));
+
+  // Return type of std::visit.
+  template<typename _Visitor, typename... _Variants>
+    using __visit_result_t
+      = invoke_result_t<_Visitor, __get_t<0, _Variants>...>;
+
+  template<typename _Tp, typename... _Types>
+    constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
+
+  template <typename _Visitor, typename _Variant, size_t... _Idxs>
+    constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
+    {
+      return __same_types<
+       invoke_result_t<_Visitor, __get_t<_Idxs, _Variant>>...
+       >;
+    }
+
 } // namespace __variant
 } // namespace __detail
 
@@ -1248,7 +1267,8 @@ namespace __variant
 #endif
 
   template<typename _Visitor, typename... _Variants>
-    constexpr decltype(auto) visit(_Visitor&&, _Variants&&...);
+    constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
+    visit(_Visitor&&, _Variants&&...);
 
   template<typename... _Types>
     inline enable_if_t<(is_move_constructible_v<_Types> && ...)
@@ -1715,41 +1735,21 @@ namespace __variant
                           std::forward<_Variants>(__variants)...);
     }
 
-  template<typename _Tp, typename... _Types>
-     constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
-
-  template <size_t _Idx, typename _Visitor, typename _Variant>
-    decltype(auto)
-    __check_visitor_result(_Visitor&& __vis, _Variant&& __variant)
-    {
-      return std::__invoke(std::forward<_Visitor>(__vis),
-                          std::get<_Idx>(std::forward<_Variant>(__variant)));
-    }
-
-  template <typename _Visitor, typename _Variant, size_t... _Idxs>
-    constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
-    {
-      return __same_types<decltype(__check_visitor_result<_Idxs>(
-       std::declval<_Visitor>(),
-       std::declval<_Variant>()))...>;
-    }
-
-
   template<typename _Visitor, typename... _Variants>
-    constexpr decltype(auto)
+    constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
     visit(_Visitor&& __visitor, _Variants&&... __variants)
     {
       if ((__variants.valueless_by_exception() || ...))
        __throw_bad_variant_access("std::visit: variant is valueless");
 
-      using _Result_type = std::invoke_result_t<_Visitor,
-       decltype(std::get<0>(std::declval<_Variants>()))...>;
+      using _Result_type
+       = __detail::__variant::__visit_result_t<_Visitor, _Variants...>;
 
       using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;
 
       if constexpr (sizeof...(_Variants) == 1)
        {
-         constexpr bool __visit_rettypes_match =
+         constexpr bool __visit_rettypes_match = __detail::__variant::
            __check_visitor_results<_Visitor, _Variants...>(
              std::make_index_sequence<
                std::variant_size<remove_reference_t<_Variants>...>::value>());
diff --git a/libstdc++-v3/testsuite/20_util/variant/100384.cc 
b/libstdc++-v3/testsuite/20_util/variant/100384.cc
new file mode 100644
index 00000000000..4866aa017ff
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/100384.cc
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++17 } }
+
+#include <variant>
+
+int visit(int*, std::true_type) { return 0; }
+
+const std::true_type dat;
+
+int i = visit(nullptr, dat);