You're confusing long double with size_t. I did a cursory look up msvc++ mangling, and long double is always 'O'. The itanium spec says that long double is 'e' - unless 128bits in which case it is 'g'.
Yes, so msvc++ has two 64-bit floating types with different manglings, and D has one. D does not have a type that can be used to match long double's mangling with msvc.
