std::visit for variant fails to compile under clang 5 [duplicate]

问题

This question already has an answer here:

get<string> for variants fail under clang++ but not g++ (1 answer)

Closed last year.

The following uses of std::visit compiles properly under gcc 7.2 but fails to compile under clang 5.0. Does anyone know what the problem is?

#include <variant>

struct S1 {int foo() { return 0; }};
struct S2 {int foo() { return 1; }};

using V = std::variant<S1, S2>;

int  bar() {
  V v;
  return std::visit([](auto& s) { return s.foo(); }, v);
}

The first error is this:

include/c++/7.2.0/variant:238:46: error: cannot cast 'std::variant<S1, S2>' to its private base class
  'std::__detail::__variant::_Variant_storage<true, S1, S2>'
  return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);

Here is a link to godbolt shows this error: https://godbolt.org/g/5iaKUm

回答1:

This is known bug 33222 that seems to only affect libstdc++'s std::variant (and other constructs using the same combination). The problem is related to friend function to templates - see the thread for more detail.

The variant from libc++ doesn't seem to use the technique of friends that libstdc++ used, so you might want to temporarily change to libc++ in the meantime.

Small update: This has been fixed in the latest trunk.

来源：https://stackoverflow.com/questions/46859053/stdvisit-for-variant-fails-to-compile-under-clang-5