libstdc++: Fix visitor return type diagnostics [PR97499]

libstdc++: Fix visitor return type diagnostics [PR97499]

[Ville Voutilainen]

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Tested on Linux-PPC64. I haven't tested this with clang yet,
Jonathan, can you help with that? The previous implementation
indeed made an if-constexpr branch invalid for all instantiations
of that branch, this one doesn't - now we have just a dependent static_assert
which is well-formed for correct visitors and ill-formed for incorrect
visitors.

2020-10-16  Ville Voutilainen  <ville.voutilainen@gmail.com>

    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.

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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};

Re: libstdc++: Fix visitor return type diagnostics [PR97499]

[Jonathan Wakely]

On 16/10/20 10:26 +0300, Ville Voutilainen via Libstdc++ wrote:
>Tested on Linux-PPC64. I haven't tested this with clang yet,
>Jonathan, can you help with that? The previous implementation
>indeed made an if-constexpr branch invalid for all instantiations
>of that branch, this one doesn't - now we have just a dependent static_assert
>which is well-formed for correct visitors and ill-formed for incorrect
>visitors.

All I checked is that <variant> can now be included using -std=c++17,
but that works with Clang.

OK for trunk, thanks.

Re: libstdc++: Fix visitor return type diagnostics [PR97499]

[Ville Voutilainen]

On Fri, 16 Oct 2020 at 13:02, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 16/10/20 10:26 +0300, Ville Voutilainen via Libstdc++ wrote:
> >Tested on Linux-PPC64. I haven't tested this with clang yet,
> >Jonathan, can you help with that? The previous implementation
> >indeed made an if-constexpr branch invalid for all instantiations
> >of that branch, this one doesn't - now we have just a dependent static_assert
> >which is well-formed for correct visitors and ill-formed for incorrect
> >visitors.
>
> All I checked is that <variant> can now be included using -std=c++17,
> but that works with Clang.
>
> OK for trunk, thanks.

The subject line is wrong, this is 97449, not 97499. Fixing that and pushing.