[swift-evolution] Warning for vacuously complying to an all-optional protocol.

William Shipley wjs at mac.com
Mon Apr 25 17:37:52 CDT 2016

The Swift 2.2 manual says:


Strictly speaking, you can write a custom class that conforms to CounterDataSource without implementing either protocol requirement. They are both optional, after all. Although technically allowed, this wouldn’t make for a very good data source.

I think it’d be helpful to get a compiler warning when you declare that a class or struct complies to a protocol (that only has optional methods) but doesn’t actually implement any of the protocol.

E.G., this show show a warning:

import Cocoa

class kindaUseless: NSObject, NSTableViewDelegate {

While this behavior is mathematically consistent, it’s not rational and doesn’t really fit in with Swift.

Extraneous protocol conformance is messy and I can’t imagine a scenario in which it’s actually useful.



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