stderr:
<stdin>:7:46: warning: redundant conformance constraint 'U': 'P' func foo<U, T>(_: U, _: T) where U : C<T>, U : P {} ^ <stdin>:7:36: note: conformance constraint 'U': 'P' implied here func foo<U, T>(_: U, _: T) where U : C<T>, U : P {} ^