That's a very nice addition, but it has a problem: As I'm working my way through Roxen to get it to compile with 7.7 in 7.4 compat mode, I'm adding regression tests to the testsuite where this and other new errors are causing previously working pike code to not compile anymore.

I think that every new error added to the compiler needs a compat check so that it's turned off in old code. It seems to me that the lack of such compat checks is systematic.