Signed integer overflow is decidedly not implementation-defined behavior in GCC, clang, recent MSVC, ... I think you misunderstand the meaning of "implementati

Signed integer overflow is decidedly not implementation-defined behavior in GCC, clang, recent MSVC, ... I think you misunderstand the meaning of "implementation-defined behavior". It means the implementation chooses a particular behavior and documents it does so. For example, a compiler might choose to always wrap on signed overflow and put that in their manual. That's not what's happening.