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

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D125919#3569928 <https://reviews.llvm.org/D125919#3569928>, @aaron.ballman wrote:

> In D125919#3560523 <https://reviews.llvm.org/D125919#3560523>, @aaron.ballman wrote:
>
>> All that said, I think you can see why I'm hoping to get an answer from WG14 as to what to do. Reasonable folks are disagreeing on what the standard requires here.
>
> The discussion on the WG14 reflector seems to be settling down to a consensus position that the `_Atomic` qualifier is only syntactically a qualifier and its use designates an entirely new type. When the standard says "unqualified type", the `_Atomic` is not considered a qualification. So we should *not* be stripping the `_Atomic` as I was doing in this patch. (SC22WG14.22200 has most of the details spelled out nicely, if you have access to the reflectors.)

What a strange position.  *All* qualifiers produce an "entirely new type".  But okay, if the committee wants to pretend that `_Atomic` is meaningful in return values, I guess we all have to live with that.

> I had asked other questions in related areas that also got answers.
>
> `const void func(volatile void);` -- the return type is adjusted to `void`; the parameter type is UB (by lack of specification) and we can do what we want here. We currently diagnose the parameter as being invalid: https://godbolt.org/z/9c8bTrerY. Our behavior with the parameter is consistent with GCC and EDG.
>
> `const int main(void)` -- this is valid and equivalent to `int main(void)`, so it should be accepted; we currently reject: https://godbolt.org/z/v43h596ev
>
> `const int func(void); int func(void) { }` -- this is DR423. It is valid, the composite type is `int(void)`; we give a conflicting types error: https://godbolt.org/z/Yb841r7Ex

Is that only because of the type compositing rules, or are they saying that the type of the first is `const int (void)`, which is different from `int (void)`?


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