The `keywordsMissingValue` argument to `Parse()` is now needed only for
the `cmake_parse_arguments` result `_KEYWORDS_MISSING_VALUES`. Offer
its implementation a private binding for this. Our internal clients can
use `ArgumentParser::NonEmpty<>` and friends to enforce the presence of
values.
Since commit f46b2e9142 (cmArgumentParser: Model maybe-missing string
with wrapper type, 2022-07-06) we know during parsing whether or not it
is an error for a keyword to be missing a value. Record such errors in
the parse results structure. Offer clients a helper method to report
them. This provides clients with an alternative to manually checking
`keywordsMissingValue` and generating their own error message.
Bindings to `std::string` require one value. Some clients have been
filtering `keywordsMissingValue` to support keywords that tolerate a
missing value. Offer them a type-safe way to achieve this instead.
Previously bindings to `std::vector<std::string>` required at least one
value. Some clients have been filtering `keywordsMissingValue` to
support keywords followed by empty lists. Instead, require clients to
specify whether a keyword's list can be empty as part of the binding
type.
Clang `-Wcomma` warns:
```
Source/cmArgumentParser.cxx:58:42: warning: possible misuse of comma operator
this->CurrentList = (val.emplace_back(), &val.back());
^
```
This was introduced by commit 4359fe133b (Introduce cmArgumentParser,
2019-03-23). Suppress it with the suggested cast.
