What would break if class was merged with struct

[Ola Fosheim Grøstad via Digitalmars-d]

On Saturday, 27 May 2017 at 16:04:42 UTC, Stanislav Blinov wrote:
> It's not "a little bit", it's ABI down the drain, along with a 
> good part of runtime. And we'd lose interfaces and extern(C++).

Why would you loose interfaces and extern(C++)?

"struct" and "class" are interchangeable keywords in C++ (only 
affects encapsulation). So not a lot more difference than POD vs 
non-POD in C++?

> That being said, it is totally possible today to have struct 
> wrappers over classes, both malloc'ed and stack-allocated.

Well, I am asking more out of a theoretical interest for what the 
core language in D could boil down to.