Explicitly allow argument markers to be assigned. Make use of this in
parsing. This simplifies working with these types, by allowing them to
be assigned from the underlying types using simple `a = b` syntax,
rather than requiring gymnastics to access the underlying assignment
operations. It also makes assignment more consistent with
initialization.
Tweak cmArgumentParser to ignore positional arguments once a keyword
argument has been seen. This prevents mingling of keyword arguments
being able to effectively skip positional arguments, with later
arguments being picked up again; this seems highly likely to lead to
user confusion. This is also consistent with how other languages (e.g.
Python) handle a mix of "named" and positional arguments.
Some clients have been explicitly checking whether the string specified
after a keyword is empty. Offer them a way to specify that the string
must be non-empty as part of the binding type.
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.
