[PATCH] D125919: Drop qualifiers from return types in C (DR423)

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D125919#3554233 <https://reviews.llvm.org/D125919#3554233>, @rjmccall wrote:

> In D125919#3554195 <https://reviews.llvm.org/D125919#3554195>, @aaron.ballman wrote:
>
>> However, I reverted the changes in this patch in c745f2ce6c03bc6d1e59cac69cc15923d4400191 <https://reviews.llvm.org/rGc745f2ce6c03bc6d1e59cac69cc15923d4400191> as I don't think they're correct. I've got some open questions on the WG14 reflectors regarding this function type rewriting exercise, but I think my dropping of the `_Atomic` qualifier is likely wrong in this patch and the fact that we're losing source fidelity in the AST is definitely an issue. The subject of https://github.com/llvm/llvm-project/issues/39595 was the behavior of the `_Atomic` specifier in a `_Generic` selection; when I realized we don't fully implement DR423, I mistakenly connected the issue with the DR. But now that I no longer think the `_Atomic` qualifier should be dropped as I was doing, these changes really don't address the issue in `_Generic`.
>
> The source fidelity issue seems like it could be solved with `AdjustedType`.  What's the issue with `_Generic`?

We get this wrong:

  _Atomic int f(void);
  
  int main(void) {
    _Generic(f(), int: 0); // Error, no matching association
  }

The controlling expression has to undergo lvalue conversion, which drops the `_Atomic` qualifier per C2x 6.3.2.1p2, so we should match the only association rather than error. We do perform the lvalue conversion: https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaExpr.cpp#L1650 but for whatever reason, we're not matching its type to the association type.


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  https://reviews.llvm.org/D125919/new/

https://reviews.llvm.org/D125919