jordan_rose added a comment.
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.
Repository:
rC Clang
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https://reviews.llvm.org/D65256/new/
https://reviews.llvm.org/D65256
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