The price of not requiring fn is mandatory forward declarations, header files, and a slower parser (because then the syntax requires knowing whether every symbol is a type or a function/variable to be parsed correctly)

I think that trade-off is absolutely not worth it. I'll take order-independent declarations and fast modules over strictly sticking to C syntax any day.

It has many usability benefits: https://c3.handmade.network/blog/p/8886-why_does_c3_use_%252...