There is ABI compatibility insofar that Carbon will be able to integrate C++ at the AST level within the compiler. This is quite smart as you are basically get

There is ABI compatibility insofar that Carbon will be able to integrate C++ at the AST level within the compiler. This is quite smart as you are basically getting ABI compatibility for free. Assuming you have the source code to the C++. I'm not sure how it will work with dynamic or proprietary libraries, but as I understand it that is already a technically unsolved ABI problem with C++.