[PATCH] D106577: [clang] Define __STDC_ISO_10646__

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D106577#2906542 <https://reviews.llvm.org/D106577#2906542>, @rsmith wrote:

> If Aaron's checked with WG14 and the intent is for this to only constrain how literals are represented, and not the complete implementation, then I'm entirely fine with us defining the macro ourselves. But that's not the interpretation that several other vendors have taken. If we're confident that the intent is just that this macro lists (effectively) the latest version of the Unicode standard that we've heard of, we should let the various libc vendors that currently define the macro know that they're doing it wrong and the definition belongs in the compiler.

The discussion on the WG14 reflectors has wound down. Summarizing the key point of the discussion, Richard asked:

> One specific example I'd like to be considered:
> Suppose the C standard library implementation's mbstowcs converts a certain multi-byte character C to somewhere in the Unicode private use area, because Unicode version N doesn't have a corresponding character. Suppose further that the compiler is aware of Unicode version N+1, in which a character corresponding to C was added. Is an implementation formed by that combination of compiler and standard library, that defines `__STDC_ISO_10646__` to N+1, conforming? Or is it non-conforming because it represents character C as something other than the corresponding short name from Unicode version N+1?

And David Keaton (long-time WG14 member and current convener) replied:

> Yikes!  It does indeed sound like the library would affect the value of `__STDC_ISO_10646__` in that case.  Thanks for clarifying the details.

There was no further discussion after that point, so I think the unofficial WG14 stance is that the compiler and the library need to collude on setting the value of that macro.


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