`<regex>`: `regex_traits<_Elem>` uses an inadmissible value of type `char_class_type` to represent character class "w"

regex_traits<_Elem> uses static_cast<ctype_base::mask>(-1) to represent the character class "w":

https://github.com/microsoft/STL/blob/8124540f8bce3faad76a6dddd050f9a69af4b87d/stl/inc/regex#L354

This is an inadmissible choice, because it violates [re.grammar]/9:

The results from multiple calls to traits_inst.lookup_classname can be bitwise or'ed together and subsequently passed to traits_inst.isctype.

Specifically, or'ing the char_class_type for "w" with the char_class_type for any other character class always produces the value for "w" again, even if the combination should match more characters.

Additional remarks

I think resolving this issue will break ABI. However, it should be possible to mitigate the problems caused by this issue.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with stl/inc/regex around the referenced regex_traits<_Elem> implementation and read [re.grammar]/9. Determine how character-class values should combine without violating the ABI, with the vNext branch and #169 as context; done means combined class lookups produce the required matches and compatibility concerns are addressed.