CTAD fails on nested templated class

[narutse]

Bugzilla Link	46856
Version	10.0
OS	Linux
CC	@zygoloid

Extended Description

The following snippet doesn't compile on clang (10), due to CTAD failing. It occurs when the parent class is templated:

template <class U>
struct X {	
	template <class T> struct Y {
		T y;
		Y(T t) : y(t) {}		
	};
	auto test() const {
		return Y(10);
	}
};

auto test() {
    return X<int>().test();
}

The snippet compiles on msvc and gcc:
https://godbolt.org/z/Wq6Paf

[AaronBallman]

This appears to be https://wg21.link/cwg2471

[llvmbot]

@llvm/issue-subscribers-clang-frontend