[PATCH] D122895: [C89/C2x] Improve diagnostics around strict prototypes in C

Fri May 13 04:44:23 PDT 2022

aaron.ballman added a comment.

In D122895#3510165 <https://reviews.llvm.org/D122895#3510165>, @dexonsmith wrote:

> For additional context to my questions above, even though open source clang hasn't been using `-Wstrict-prototypes`, Xcode has had it on-by-default in new projects since sometime in 2017, with project modernizations to turn it on for old projects.

Thanks, that's very good to know! And also, thank you for raising the questions here, I appreciate the discussion.

> Warning on block and function definitions such as `^(){}` and `void f1() {}`, which are pedantically lacking a prototype but the compiler knows there are exactly zero parameters, would be super noisy for users.
>
> Unless I read the RFC too quickly, it doesn't look like it's adding any value, since these aren't going to change meaning. Is it possible to revert this part of the change?

`-Wstrict-prototypes` is now a pedantic deprecation warning that fires any time you form a function type which has no prototype, which was discussed in the RFC (https://discourse.llvm.org/t/rfc-enabling-wstrict-prototypes-by-default-in-c/60521/38?u=aaronballman):

  Change -Wstrict-prototypes to diagnose functions without a prototype that don’t change behavior in C2x, it remains off-by-default but is automatically enabled by -pedantic as it’s still warning the user about a deprecation.

However, I think the blocks behavior is a bug based on this: https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaType.cpp#L728 and I'd be more than happy to fix that, as I'm not checking that condition here: https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaType.cpp#L5579 which seems like a pretty obvious thing to be checking for before warning about not having a strict prototype.

But I'm pretty insistent on warning about the other case as it does use functions without a prototype and we need *some* blanket warning for use of a deprecated feature for folks who want to be strictly conforming. It sounds like Apple may want to no longer enable `-Wstrict-prototypes` by default as it's shifted to be a more pedantic warning than it was before -- would that be a viable option for you (can you use project modernizations to turn it back off for old projects)?

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https://reviews.llvm.org/D122895