[clang] [Sema] Diagnose use of if/else-if condition variable inside else-if/else branch(s) (PR #156436)

[via cfe-commits]

https://github.com/Sirraide commented:

So from a high-level point of view, I definitely think there is value in having this because I personally have made this mistake *so* many times, and honestly, I think we should just straight-up error on it (or at least make it a warning that defaults to an error) because I cannot think of a situation where doing this would ever be valid.

That said, using an AST visitor for this is... probably not great because we now do an AST traversal on every `else` branch ever, which sounds like it might hurt performance quite a bit—that said, this is pretty much my only concern w/ this patch, so if it turns out that this isn’t the case at all then this approach is fine I suppose.

CC @nikic could you put this on llvm-compile-time-tracker so we know what the overhead of this is if any?

At the same time, I’m not sure I can think of a better way to do this. I suppose another option might be to add a flag to `VarDecl` that keeps track of whether it’s declared in the condition of an `if` statement and then somehow have scope information to figure out where a given use of it is relative to that `if` statement, but that sounds like it’d get a bit complicated, and I’m not sure running this sort of check on every DRE would be any better...

(Side note: sometimes when there are performance considerations we prefer to move these sorts of diagnostics into the static analyser, but I think this is situation is bad enough to where we want to do this in the compiler proper—at least I can think of many instances where this would have helped me quite a bit.)


https://github.com/llvm/llvm-project/pull/156436