> IMHO the current main problem is that the C stdlib is stuck in the K&R era and the stdlib APIs haven't even been updated to the language features added in C99 (e.g. make use of struct args and return values).

C++ has the same issue (only with more chaos and bloat). They add some new good idea (like optional) but don't update the rest of the standard library to make use of it. And they can't really, without breaking backwards compatibility.

I think looking at the edition system in Rust could be useful for C and C++ to start to solve this. Something like "If this source file has this pragma in it, compile that code with a new edition". It would have to be granular, per expression really, to handle macros (which is how it works in Rust too). What would it change in C/C++? Name resolution, you could get a different set of resolvable overloads, depending on which edition is active in the caller context. Not unlike an enable_if.

The same would work in C: depending on the caller edition, expose function signatures.