[Patch] Add C++14 feature querying macro `cxx_digit_separators` describing support for "Single-Quotation-Mark as a Digit Separator"

[Richard Smith]

On Tue, Oct 21, 2014 at 1:04 AM, Michel Morin <mimomorin at gmail.com> wrote:

> The support for C++14 digit separators was already added in Clang 3.4.
> But there is no querying macro for the feature.
>
> I've attached a patch for adding the macro.


The patch looks good to me, but I have a more general question before we go
ahead with this: should we continue to add Clang-specific __has_feature
tests for features that are covered by C++'s SD-6 feature test
recommendations? Per the latest working draft of SD-6, the portable,
cross-compiler way to check for digit separators is

  #if __cpp_digit_separators >= 201309L

... and there seems to be no reason for portable code to use __has_feature
for this check.

(That said, I *can* see an argument for supporting __has_extension for such
features in general... though perhaps not for this one, since it would not
be a conforming extension in C++11.)
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