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.voutilai...@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.
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};
