Comment by fweimer

It's a hard problem because many ABIs cannot do tail calls even for very basic stuff, like calls to extern functions with matching argument and return types. It looks like Clang has some heuristics for changing call sequences for musttail calls. For example, it switches to noplt calls on i686. None of this is mentioned in the Clang documentation: https://clang.llvm.org/docs/AttributeReference.html#musttail

What's realistic here is that you get a diagnostic if the compiler cannot generate a tail call. For many users, that will likely be good enough. Guaranteeing a tail call as in Scheme is unlikely to happen.