Converting a pointer-to-member-of-base to a pointer-to-member-of-derived

Question

A simplified example from a recent blog post:

struct B { void f(); };
struct D : B { };

constexpr auto as_d = static_cast<void(D::*)()>(&D::f); // (1)

template <void (D::*)()>
struct X { };

X<as_d> x; // (2)

gcc, clang, and MSVC all accept the declaration of as_d marked (1). gcc and clang both reject the declaration of x marked (2), but MSVC accepts it.

Both gcc's and clang's error messages indicate that they know that as_d is a pointer to member of B. clang:

<source>:9:3: error: sorry, non-type template argument of pointer-to-member type void (D::*)() that refers to member B::f of a different class is not supported yet

gcc:

<source>:9:7: error: void (D::*)(){((void (D::*)())B::f), 0} is not a valid template argument for type void (D::*)()

Who is right? If gcc/clang, what is the rule we're running afoul of? It sure seems like as_d is a converted constant expression of type void (D::*)() to me...

Answer 1

Well, this ended up being pretty interesting. As far as the language is concerned, the program is valid - as_d does meet the requirement for being a valid non-type template argument (it is a converted constant expression of the right type).

However, the Itanium C++ ABI apparently does not specify a mangling for this situation (that is, having a non-type template argument whose type is a pointer-to-member-to-derived but whose value is a pointer-to-member-to-base). Compilers targeting that ABI (i.e. clang and gcc), as a result, can't accept this code. This explains why clang's error is "sorry, not yet" rather than "no, bad!"

On the other hand, other ABIs have no such mangling problem, and so MSVC and ICC are both able to compile the program just fine.