related issues: rust-lang/rust#51445
If one crate lib.rs implements a trait(Default) for an associated type(<() as Container>::Item, which is Foo),
in another crate main.rs, it seems the compiler treats the associated type(<() as Container>::Item) as a generic type T and assumes T would be any type and results in the conflict with a local type that tries to implement Default,
while actually Foo and Wow are actually different types.
And impl Default for <() as Container>::Item does not conflict with types defined in the same crate lib.rs
