Extended Description

Clang currently rejects this program, as the standard says it is ill-formed in [conv.mem], I think:

struct A { int a; };
struct B : virtual A { int b; };
int B::*memptr_b = &B::a; // error

memptr_data_virtual.cpp:8:20: error: conversion from pointer to member of class 'A' to pointer to member of class 'B' via virtual base 'A' is not allowed
int B::*memptr_b = &B::a; // error

MSVC accepts this program, and has a whole bunch of machinery baked into the ABI to support it. Data member pointers can be formed with up to 3 (!) i32s.

There's basically no way to implement this within the Itanium ABI since data memptrs there are always just one offset, so this would only be in -fms-compatibility (or -fms-extensions, I forget which is which).

Mostly I'm just filing this to have a discussion of record with a decision to implement or not. We should document this as part of http://clang.llvm.org/compatibility.html, probably.