> gcc and clang can't change their behavior because that would be a breaking ABI change

It's an API change. Breaking source code is generally an even bigger deal than breaking binary compatibility.

Eh, both are about the same? With the source-level API changes, you can patch the source code or, you know, just keep using the already existing binaries, or keep using the old compilers. But if you break ABI, you can't keep using those already existing binaries, you need to start upgrading in bulk. Using the new compilers. That also probably have some source-level API changes. Oh dear.