[SUGGESTION] functions/lambdas in requires

[realgdman]

Not full issue, just several observations

foo: () -> bool = true;
S: <T: type> type one of requires below = { }

requires foo()
requires (foo())

1. I haven't found way to write constexpr function in cpp2 which is needed here.
   This could be separate issue.
2. Imagine foo is constexpr. But currently there is following cpp1 error:

error: use of undeclared identifier 'foo'
error: requires clause differs in template redeclaration

Because emitted declarations in that order:

template<typename T> requires( (foo()) ) class S //undeclared foo
auto foo() -> bool;
template<typename T> requires( (foo()) ) class S //template redeclaration
auto foo() -> bool { return true;  }

As I understand classes declared first, so we can refer them in functions, but in this case its opposite, class declaration relies on function.

3. Immediate lambda invocation

	requires :(x)=true;(0)
	requires :(x)->_=true;(0)
	requires (:(x)->_=true;(0))

All those give cpp2 parse error

error: unexpected end of source file

Additional info, example of cpp1 requires with function and lambda
https://godbolt.org/z/a48YePnxe

[JohelEGP]

See also #323, #385.