P2996 - `is_consteval_only_type` is not so that powerful as implied

[frederick-vs-ja]

When investigating the recent changes (240e3a2) in P2996, I found that such ill-formedness can be reproduced in C++20 (patched by P2564R3) without actually consteval-only types, see the test cases in llvm/llvm-project#134623.

The crux is that in C++20 we can write iterators with (possibly partially) consteval-only operations, which triggers the same errors with the current implementation strategy of libc++. And we can't just dispatch such cases with is_consteval_only_type.

Still, is_consteval_only_type is perhaps somehow useful, but unfortunately not so in sort.

[brevzin]

Ugh. That's... a really good observation.