Pointer to data member of incomplete type

Question

Is the following code well-defined in C++? (*)

I'm having a hard time figuring out where to even look in the standard, and searching the web hasn't turned up anything concrete.

struct S;
struct T {
    constexpr T() = default;
    bool S::* a = nullptr;
    int b       = 42;
};
const T t{};
// Test. Compiled using: cl /W4 /WX /FAs filename.cpp
#include <stdlib.h>
int main() {
    if (t.b != 42) abort();
}

The reason I'm asking is because it works (or seems to) with newer versions of GCC and Clang (x86/x86_64), but fails(**) with Visual Studio 2015 Update 2 and Update 3 RC.

Before reporting a bug I'd like to be sure I'm not relying on undefined behavior or just not searching for the right terms.

I've also tried using /vmg and /vmb as mentioned in this post.

(*): I mostly care about C++14 and later, but I don't see any reason the answer shouldn't apply to C++11.
(**): If the code is well-defined it looks like a codegen bug where it isn't allocation room for the pointer. Changing struct S to struct S{} seems to make the code "work".

Answer 1

Your code is well-defined:

N4594 3.2/5

[...]A class type T must be complete if:

• (5.1) an object of type T is defined (3.1), or
• (5.2) a non-static class data member of type T is declared (9.2), or
• (5.3) T is used as the object type or array element type in a new-expression (5.3.4), or
• (5.4) an lvalue-to-rvalue conversion is applied to a glvalue referring to an object of type T (4.1), or
• (5.5) an expression is converted (either implicitly or explicitly) to type T (Clause 4, 5.2.3, 5.2.7, 5.2.9, 5.4), or
• (5.6) an expression that is not a null pointer constant, and has type other than cv void*, is converted to the type pointer to T or reference to T using a standard conversion (Clause 4), a dynamic_cast (5.2.7) or a static_cast (5.2.9), or
• (5.7) a class member access operator is applied to an expression of type T (5.2.5), or
• (5.8) the typeid operator (5.2.8) or the sizeof operator (5.3.3) is applied to an operand of type T, or
• (5.9) a function with a return type or argument type of type T is defined (3.1) or called (5.2.2), or
• (5.10) a class with a base class of type T is defined (Clause 10), or
• (5.11) an lvalue of type T is assigned to (5.18), or
• (5.12) the type T is the subject of an alignof expression (5.3.6), or
• (5.13) an exception-declaration has type T, reference to T, or pointer to T (15.3).

None of them says that T needs to be complete in order to declare pointer-to-member of T.