Fortunately, C++ taketh away and C++ giveth, both, so here we are: a way to
detect if we're in the middle of definining a type that uses itself as a
template parameter in a way that flows into a concept check and, if so,
short-circuit out of the need to actually do any checks. Wonders never cease.
This will not be used unless the C++ standard version is raised to 20. As
concepts and C++20 more generally are quite new, this does not do so.
Nevertheless, the use of concepts can improve the local development experience
as type mismatches are discovered earlier (at template invocation rather than
only during expansion).
