stderr:
<stdin>:6:36: warning: redundant conformance constraint 'U': 'P' func foo<U, T>(_: U, _: T) where U : P, U : C<T> {} ^ <stdin>:6:43: note: conformance constraint 'U': 'P' implied here func foo<U, T>(_: U, _: T) where U : P, U : C<T> {} ^