[gcc r11-4024] libstdc++: Fix visitor return type diagnostics [PR97449]

[Ville Voutilainen]

https://gcc.gnu.org/g:1f65bf2aa65609c0cd88af1b83191d37d6729f46

commit r11-4024-g1f65bf2aa65609c0cd88af1b83191d37d6729f46
Author: Ville Voutilainen <ville.voutilainen@gmail.com>
Date:   Sat Oct 17 22:08:50 2020 +0300

    libstdc++: Fix visitor return type diagnostics [PR97449]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/97449
            * include/std/variant
            (__gen_vtable_impl<>::_S_apply_single_alt):
            Diagnose visitor return type mismatches here..
            (__gen_vtable_impl</*base case*/>::_S_apply):
            ..not here.

Diff:
---
 libstdc++-v3/include/std/variant | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index a29c5bf513b..17f8bcd638b 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -960,9 +960,13 @@ namespace __variant
 	    }
 	  else
 	    {
-	      __element = __gen_vtable_impl<
+	      auto __tmp_element = __gen_vtable_impl<
 		remove_reference_t<decltype(__element)>,
 		std::index_sequence<__indices..., __index>>::_S_apply();
+	      static_assert(is_same_v<_Tp, decltype(__tmp_element)>,
+			    "std::visit requires the visitor to have the same "
+			    "return type for all alternatives of a variant");
+	      __element = __tmp_element;
 	    }
 	}
     };
@@ -1026,10 +1030,8 @@ namespace __variant
 				    std::declval<_Variants>()...))>;
 	    if constexpr (__visit_ret_type_mismatch)
 	      {
-		static_assert(!__visit_ret_type_mismatch,
-		  "std::visit requires the visitor to have the same "
-		  "return type for all alternatives of a variant");
-		return __nonesuch{};
+		struct __cannot_match {};
+		return __cannot_match{};
 	      }
 	    else
 	      return _Array_type{&__visit_invoke};