Whenever I mentally parse `protocol SomeProtocol: SomeOtherProtocol`, it’s meant to imply `SomeProtocol` adds additional requirements onto another protocol, namely `SomeOtherProtocol`’s. What’s odd about class-constraining protocols is that it’s been a weird historical inconsistency in Swift where the term to the right of the `:` _isn’t_ a protocol and instead a reserved word. All reference types implicitly conform to `AnyObject`, which, being a protocol, makes it a more consistent way of class constraining than remembering the special `: class` trick. This merging of concepts was lightly mentioned in [SE-0156](https://github.com/apple/swift-evolution/blame/93abb54833e2d9ee7ee842882f6104a867de3069/proposals/0156-subclass-existentials.md#L134) (link to specific line).