[PATCH] D65256: [Sema][ObjC] Mark C union fields that have non-trivial ObjC ownership qualifications as unavailable if the union is declared in a system header

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D65256#1601415 <https://reviews.llvm.org/D65256#1601415>, @jordan_rose wrote:

> In D65256#1601410 <https://reviews.llvm.org/D65256#1601410>, @rjmccall wrote:
>
> > These were unavailable in system headers before because otherwise we would've had to make them invalid.  Since these unions are no longer otherwise invalid, there shouldn't be a problem with allowing them in system headers, and in fact making the semantics vary that way seems quite problematic.  Now, specific *uses* in system headers might still appear to be invalid — e.g. an ObjC ivar of type `union { __strong id x; }` — and the right behavior is definitely that those use sites should be marked as invalid instead of refusing to compile the system header.
>
>
> That's the right answer on paper, but it's source-breaking in practice, for both Swift and Objective-C. On the Objective-C side, someone could have been using, say, `glob_t` in a copyable way in their ARC code, never touching the block field, and it would have been working fine. On the Swift side, we won't be able to import such a union at all when previously we would have.


Sorry, am I missing something?  Such a union would've been either ill-formed or unavailable in ARC (depending on where it was declared) before this recent work.


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https://reviews.llvm.org/D65256