Initialization of nested struct fields

[Peter Alexander via Digitalmars-d-learn]

Can someone please explain this behaviour? I find it totally 
bizarre.

auto f(T)(T x) {
	struct S {
		T y;
		this(int) { }
	}
	return S(0);
}


void main() {
	f(f(0));
}

Error: constructor f376.f!(S).f.S.this field y must be 
initialized in constructor, because it is nested struct

Why must y be initialized in the constructor? It isn't const. Why 
isn't it default initialized?

Is this explained anywhere in the docs? I can't see anything in 
the nested struct section, or in any constructor section.