erichkeane added a comment.

In D150226#4408381 <https://reviews.llvm.org/D150226#4408381>, @aaron.ballman 
wrote:

> In D150226#4404808 <https://reviews.llvm.org/D150226#4404808>, @rupprecht 
> wrote:
>
>> I suppose including this warning in `ShowInSystemHeader` with your diff 
>> above would be a good first step anyway, right? If we're about to make it a 
>> hard error anyway, then making it a `ShowInSystemHeader` error first would 
>> ease that transition.
>
> Yes and no.
>
> If we're going to turn something into a hard error, letting folks 
> implementing system headers know about it is important, so from that 
> perspective, it makes sense to enable the diagnostic in system headers. 
> However, *users* have no choice in their system headers (oftentimes) which 
> makes the diagnostic unactionable for them as they're not going to (and 
> shouldn't have to) modify system headers, so from that perspective, enabling 
> the diagnostic in a system header introduces friction.
>
> If this diagnostic is showing up in system headers, I think we would likely 
> need to consider adding a compatibility hack to allow Clang to still consume 
> that system header while erroring when outside of that system header (we've 
> done this before to keep STL implementations working, for example). Between 
> this need and the friction it causes users to have an unactionable 
> diagnostic, I think we probably should not enable `ShowInSystemHeader`.

It seems to me that if our concern is breaking system headers, we need to do 
that with better testing.  Some sort of 'diagnostic group' for "This is going 
to become an error *SOON*" mixed with us/vendors running that on platforms we 
consider significant enough to not break.  But just diagnosing on arbitrary 
users with no choice on how to fix the headers doesn't seem appropriate.


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