Why do we prohibit redundant declarations of conformance to a protocol?

[tevelee (Laszlo Teveli)]

FYI Slava recently pointed this out on the noncopyable generics thread.

@Slava_Pestov

On the subject of redundant requirements in general:

@cal

A question about the new Copyable protocol: what happens if you try to manually write a redundant Copyable conformance? Would it make sense for this to result in an error or warning?

So back in the day (Swift 5.7 or 5.8, I forget now) we used to diagnose redundant generic requirements. This was disabled by default at some point and then I removed the code recently. So writing T: Copyable anywhere is just silently ignored. The compiler happily accepts this for example even though the entire where clause is pointless:

func f<T: Sequence>(_: T) where T: Sequence, T.Iterator: IteratorProtocol {}