[format.arg] Apply \exposid consistently

[AlisdairM]

Consistently apply the \exposid macro when typesetting exposition-only private data members and typedef-name members in basic_format_arg and related parts.

Note that I confirmed the exposition-only intent against P0645R10 to ensure that the use of code font was a genuine misapplication of the original paper.

[jwakely]

I think you missed one, but otherwise I agree with the change. For libstdc++ I used this instead of the "not exposition-y" char_type:

using _CharT = typename _Context::char_type;

[jwakely]

I don't think we have agreement on adding underscores to data member names, i.e. value vs value_. There is no value() member function that it could clash with, so value should be ok.

[JohelEGP]

Underscores are discussed at #4702.

[jwakely]

Underscores are discussed at #4702.

Indeed, and given the general dislike of the underscores and the strong objection, I don't think adding them here is progress. At the very least, controversial.

[tkoeppe]

That's right, the naming of exposition-only variables is a delicate matter that we should not change as a drive-by.

[AlisdairM]

While I strongly agree with the distaste for trailing underscores, I think this case merits special attention. value is the only exposition only private data member in the format framework that does not have a trailing underscore, unless it has internal dashes for kebab case to indicate that it is an exposition-only member. See args_, begin_, data_, end_, format_, indexing_, next_arg_id_, num_args_, out_, ptr_, separator_, size_, underlying_, and even opening-bracket_ and closing-bracket_.

On the other hand, I find just str in basic_format_string (so I was wrong about value being the only example).

I suggest making value_, and possibly str_, consistent through this facility, and then tackling the whole issue if we want to deal with trailing underscores.

[tkoeppe]

I see, yes, the rest of the formatter spec already uses underscores, so changing these two does indeed seem like an improvement. Go for it.

[frederick-vs-ja]

@AlisdairM Ping. This needs to be rewritten since [format.arg] is moved to text.tex.

[frederick-vs-ja]

I created a "manually rebased" version of this (#8971).