Personally if I wanted to write similar functionality in standard C++ (much like fmtlib's compile-time format string checking) rather than compiler-specific C extensions, I'd try making wmemcpy a template on the T* being passed in, then check against sizeof(T). This raises several questions:

- Does this falsely raise buffer overflow errors for T with flexible array members where the actual allocation is bigger than the T?

- On a related note, is it even legal to memcpy more than a T's worth of data into a T? (IMO you shouldn't be doing that, but I think it's legal in C, and not with my template trick.)

- You can't take the address of a template function. What happens when you try to take &wmemcpy? Do you get a non-inlined wmemcpy with a failing branch which never knows the object size? A call to __builtin_dynamic_object_size? Or &__original_wmemcpy?