[kazinator]

How so? C99+ support/conformance is a compiler feature. That feature breaks/conflicts with some aspects of C90 support, an existing, older feature. So you need a feature flag. Inside comiler there are various places where you have the equivalent of "if C90 do this, else do that".

[startling]

"Feature flags" tend to be for behavior that is being developed and tested. The -ansi flag is more like configuration. It's pretty valuable to continue to support C90.