[PATCH] D111400: [Clang] Implement P2242R3

Tue Oct 26 10:23:26 PDT 2021

hubert.reinterpretcast added a comment.

In D111400#3087877 <https://reviews.llvm.org/D111400#3087877>, @aaron.ballman wrote:

> So your concern is that allowing this patch to be used as an extension in older language modes may change the behavior of existing code? Or is your concern more broad than just this patch?

The behaviour of Clang before this patch in older language modes is inconsistent with the specification before P2242R3 (for the template case) in such a way that it could appear that P2242R3 is already implemented for the template case (but not following the usual convention for extensions affecting SFINAE); however, the mechanism by which that occurs might be something other than an implementation of P2242R3.

My concern is that the status quo of the Clang implementation with respect to this area is broken, which in turn makes it possible for this patch to exacerbate the issue by building on top of the brokenness and then making a fix more complicated. At the very least, this patch does not demonstrate that the "extension" does not affect SFINAE.

In other words, my concern is that this patch is necessarily incomplete unless if the situation around the template case is resolved.

> Thank you for the example code, that helps add clarity. I think it's worth noting that implementations disagree here in a few different ways: https://godbolt.org/z/f9KnMhTGd

ICC fails to SFINAE -- but at least it is consistent with GCC in considering the expression to be non-constant.

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