Requires expressions are not allowed to be used as bool expressions

[steve02081504]

demo code: https://godbolt.org/z/6o6K9GnnY

#define was_not_an_ill_form(...) (bool(requires { __VA_ARGS__; }))
template<class T>
T declval();

struct A {};
template<class T>
constexpr bool is_func = was_not_an_ill_form(declval<T>()());

int	main() {
	static_assert(!is_func<A>);
}

work in gcc: https://godbolt.org/z/ddEYahYox

[shafik]

Removing the outer parens makes it well-formed in both clang and gcc but not sure if this should be valid with the parens or not: https://godbolt.org/z/rshn8zaEd

CC @erichkeane this diagnostic (err_requires_clause_inside_parens) comes from code you were looking at recently

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[erichkeane]

Hmm, that looks like a parse error, I don't see why that would be considered a trailing requires clause. I haven't looked into the parsing code in a while, but I'll put this on my list of things to look at.