Found two things that stood out as odd.

Case A

auto foo() -> decltype([](){ return 4; });

This defines a lambda class but then uses it like this

__lambda_5_24 foo();
() -> decltype([](){ return 4; });

Which does not seem like correct C++

Case B

using x = decltype([](){ return 4; });
auto foo() -> x;

Which should be the same as Case A but the output does seem more correct but it does not define a lambda class.

using x = decltype([](){ return 4; });
__lambda_19_20 foo();

Insights link.
https://cppinsights.io/s/51754024