[Bug c++/94128] ICE on C++20 "requires requires" with lambda

[bisqwit at iki dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94128

--- Comment #2 from Joel Yliluoma <bisqwit at iki dot fi> ---
Yes, it is valid.

— The auto parameter is valid since C++20. It is called a “placeholder type”,
which has existed since C++11. C++20 made it valid also in function parameters.

— The “requires” is a valid keyword since C++20. It specifies constraints that
the parameter must match. The double “requires” manifests in certain
situations.

— Until C++20, lambdas were not permitted in unevaluated contexts. Changed in
C++20.