Added support for requires() constraints on methods that are not directly templated

[justinboswell]

Currently, cxxheaderparser can only parse requires() constraints on methods which are also templated. However, under C++20, requires statements can also exist on un-templated methods in a templated class as well, this is usually used to replace std::enable_if<> function toggling.

[justinboswell]

pytest output:

(.venv) PS C:\dev\cxxheaderparser> python -m pytest
====================================================================================== test session starts =======================================================================================
platform win32 -- Python 3.10.11, pytest-7.4.3, pluggy-1.3.0
rootdir: C:\dev\cxxheaderparser
collected 292 items

tests\test_abv_template.py .....                                                                                                                                                            [  1%]
tests\test_attributes.py ....                                                                                                                                                               [  3%]
tests\test_class.py ..............................................                                                                                                                          [ 18%]
tests\test_class_base.py ....                                                                                                                                                               [ 20%]
tests\test_concepts.py .............                                                                                                                                                        [ 24%]
tests\test_doxygen.py ............                                                                                                                                                          [ 28%]
tests\test_enum.py ...............                                                                                                                                                          [ 33%]
tests\test_fn.py ............................                                                                                                                                               [ 43%]
tests\test_friends.py ....                                                                                                                                                                  [ 44%]
tests\test_misc.py .............                                                                                                                                                            [ 49%]
tests\test_namespaces.py ......                                                                                                                                                             [ 51%]
tests\test_operators.py ....                                                                                                                                                                [ 52%]
tests\test_preprocessor.py sssssssssssss                                                                                                                                                    [ 57%]
tests\test_skip.py ...                                                                                                                                                                      [ 58%]
tests\test_template.py ...................                                                                                                                                                  [ 64%]
tests\test_tokfmt.py ..........................                                                                                                                                             [ 73%]
tests\test_typedef.py .......................                                                                                                                                               [ 81%]
tests\test_typefmt.py .................                                                                                                                                                     [ 87%]
tests\test_union.py ..                                                                                                                                                                      [ 88%] 
tests\test_using.py ...........                                                                                                                                                             [ 91%]
tests\test_var.py ........................                                                                                                                                                  [100%]

================================================================================ 279 passed, 13 skipped in 2.05s ================================================================================= 

[virtuald]

The requires statement gets completely discarded here, whereas in other contexts the requires is retained as a raw Value? Even though we aren't doing anything with the data at the moment, this is inconsistent and could be a problem if we ever decide to do something with the data.

It seems that the best way to deal with this would be to move raw_requires_post into Function instead of being in TemplateDecl? Even though that would be a breaking change, since this is mostly unparsed anyways I don't think it's that big of a deal.

[justinboswell]

It's not entirely discarded, right? It ends up as the raw_requires_post on a TemplateDecl with no template args, which was the sanest way I could think to represent it at a glance.

That said, it probably does make sense to move raw_requires_post to Function. I actually looked at doing that first before I realized what the parsing issue was. I've only taken a cursory dive into the parser but it seemed initially like it might be a much bigger/far-reaching change to parsing.

Now that I've spent more time in there (working on #87), It might be doable right around where trailing return types are parsed in _parse_fn_end and _parse_method_end. Would it be preferable if I put a peek in those spots looking for requires, parse it, and store the result on Function?

If so, just let me know, and I'll take a run at it!

[virtuald]

It isn't discarded immediately, but eventually the TemplateDecl isn't used so it is discarded. If it wasn't discarded, then the requires(std::is_pod_v<T>) would be in the testcase's ParsedData somewhere, and it isn't.

When I was looking at this I noticed a simpler way to do this, so I'm going to close this in favor of doing it that way.

[virtuald]

... which was exactly what you suggested, ha

[justinboswell]

Oh sweet, thank you!

[virtuald]

Closed in favor of #89, please take a look and let me know if that works for you