Library triggers bug in MSVC++ v19 (VS 2022) under modern C++ standards, preventing compilation

[burnpanck]

When attempting to migrate our code from VS 2019 to VS 2022, we suddenly started seeing error C2065: '<some-identifier>': undeclared identifier at four places in the library, where <some-identifier> is the name of a previous function parameter appearing within FOONATHAN_SFINAE.

With the help of the people at Stack Overflow, we deduced that this must be a bug in the compiler, as proven by the minimal failing example.

A potential workaround in this case would be to replace the references to previous function arguments in SFINAE expressions with std::declval based expressions. Would that be something this project could consider, or do we hope for a compiler fix? (the long list of bug-reports doesn't make me all too hopeful though).

[foonathan]

I'm happy to workaround, but since I don't have MSVC, it's quicker if you do a PR.

[burnpanck]

Ok, I'll give it a shot. Any preferences how you would like the workaround to be implemented? Use std::declval always, use a FOONATHAN_DECLVAL instead, or conditionally select a std::declval workaround only on affected compiler versions?