[Bug c++/67342] New: Ill-formed move constructor ignored in favor of copy constructor

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67342

            Bug ID: 67342
           Summary: Ill-formed move constructor ignored in favor of copy
                    constructor
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

In the following code, A is neither movable nor copyable, and B has an A:

#include <utility>

struct A {
    A() { }
    A(A const&) = delete;
    A(A&& ) = delete;
};

struct B {
    A a;

    B() = default;
    B(B const& ) : a() { }
    B(B&& ) = default;
};

int main()
{
    B b1;
    B b2(std::move(b1));
}

I would expect the move constructor to be preferred and this code to fail to
compile (with some diagnostic relating to the deleted A&& constructor), but
instead gcc (and clang) prefers the copy constructor for b2 here and the code
compiles.

I suspect this is a compiler bug, as if I instead defaulted the copy
constructor, gcc issues a diagnostic about "error: use of deleted function
'B::B(B&&)'"