[clang] clang should have a warning to disallow re-externs (PR #109714)

[via cfe-commits]

Sirraide wrote:

Oh wait, I see this is a case of the declaration and definition not matching. That is indeed bad, but there is no good way of distinguishing this from a declaration that actually matches the definition. You could put that declaration in a header, include it in one file, and then... not do that in another file and implement a completely different function with the same name. That would run into the same problem.

I don’t know what the C term for this is, but in C++, situations like these are called IFNDR (ill-formed, **no diagnostic required**), because there is no good way of diagnosing this. This would have to be done at link time, and linkers don’t know a thing about the C type system. 

Simply diagnosing every extern declaration is not a solution for this and is going to lead to a lot of false positives—probably even mostly false positives because I’d expect that the people that actually do this take care to make sure the declaration and definition match.

https://github.com/llvm/llvm-project/pull/109714