https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113653
--- Comment #6 from Rich Felker <bugdal at aerifal dot cx> --- I'm aware of the allowance to accept "other forms". It's unfortunately underspecified (does the implementation need to be specific in what forms? document them per the normal rules for implementation-defined behavior? etc.) but indeed it exists. Regardless, at least -pedantic should diagnose this, because it's a big footgun for writing code that is not valid C, that only works with certain compilers that implement C++-like behavior in C. I would also be happy with a separate warning option controlling it, named something like like -Wextended-constant-expressions.
