[Bug c++/84866] Incorrectly instantiating move ctor when a union's move constructor is implicitly deleted

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84866

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
#include <type_traits>
#include <tuple>

// X has trivial copy ctor and nontrivial move ctor
struct X  {
    X() = default;
    X(const X&)=default;
    X(X&&) {}
};

// X needs to be nested to trigger the bug
union T1 {
    X x;
};

union S1 {
    int i;
    T1 t;
};

union T2 {
    int i;
};

// If X is in the top-level of the union, it works fine.
union S2 {
    X x;
    T2 t;
};

// Since X has nontrivial move constructor, it should cause the unions
// (S1 and S2) to have deleted move constructor and instead fallback to
// use the copy constructor. Since X has a trivial copy constructor,
// the unions (S1 and S2) should be copy constructible and
// is_move_constructible should be true for S1 and S2.
bool MoveConstructible1() {  return std::is_move_constructible<S1>();  }
bool MoveConstructible2() {  return std::is_move_constructible<S2>();  }

// However, when actually invoking the copy constructor (move is 
// deleted), GCC tries to instantiate the union's move constructor
// and give error.
S1 Move1(S1 s) {  return std::move(s);  }
S2 Move2(S2 s) {  return std::move(s);  }