[cpp.cond,cpp.predefined,version.syn] Highlight preprocessor dates

[JohelEGP]

This is a follow up for #5109. Code highlighting has been previously discussed at #572.

I have a working example at https://github.com/JohelEGP/draft/tree/highlight_cpp_dates that looks like this:

[JohelEGP]

The month highlighted in light gray:

[jensmaurer]

Probably not.

[jensmaurer]

Sorry, this was ambiguous: I was specifically referring to the "grey month" version. We should form a wider consensus whether we want any color-based highlighting here at all.

[JohelEGP]

I don't think the colors I chose do the proposal any justice. They look like hyperlinks, one unused and one used. Using bold didn't help; the code hardly looked any different.

[JohelEGP]

I find this looks much nicer:

[JohelEGP]

My suggestions are a bit hideous. It'd be better to base the issue's resolution by referencing existing documents. This is probably clash with general code highlighting anyways.

[AlisdairM]

I have previously prepared a PR that I did not push, where I used digit separators for all the long literals in the standard. In that case, these macros were rendered as:
#define __cpp_lib_addressof_constexpr. 2016'03L // also in <memory>

Is that something I should consider submitting instead?

The main intent in that PR was making the large numbers specified in the random numbers clause more readable.

[JohelEGP]

Is that something I should consider submitting instead?

No, see:

This is a follow up for #5109.

[tkoeppe]

Editorial meeting decision: we prefer if we added digit separators, and that is an observable change that requires a (small) paper (with an Annex C change). Such a paper is warmly invited.

[JohelEGP]

IIRC, the proposal to add feature test macros (I can't seem to find it)
introduced this idiomatic way to test a feature's level of conformance.
Taken from https://en.cppreference.com/w/cpp/feature_test#Example:

#if __cpp_constexpr >= 201304                // Check for a specific version of a feature
#  define CONSTEXPR constexpr
#else
#  define CONSTEXPR inline
#endif

If ' were used to split dates, this idiom would stop working.

[jwakely]

If ' were used to split dates, this idiom would stop working.

Why? The preprocessor understands digit separators and they don't affect the value.