Issue Description:

When updating from Xcode 8 to Xcode 9, my project began getting warnings of the pattern:

Unavailable instance method ‘compareQueryResult(_:toQueryResult🙂’ was used to satisfy a requirement of protocol ‘ResultComparable’.

@interface DocumentQuery: Query {
   - (ComparisonResult)compareQueryResult:(DocumentResult *)result1 toQueryResult:(DocumentResult *)result2;
}
@end

public protocol ResultComparable
{
    associatedtype ResultType
    func compareQueryResult(_ r1: ResultType, toQueryResult r2: ResultType) -> ComparisonResult
}

The issue was diagnosed by @belkadan on [swift-users]:

The name you’ll get in Swift for these methods is probably 'compareQuery(_:toQuery🙂', because the Swift compiler detects that the “result” is redundant with the type name. In Xcode 8 the compiler didn’t check if you were using the “old" name to satisfy a protocol, but Xcode 9 now does.

Since this is a source-breaking change, he asked me to file an issue at https://bugs.swift.org. Noting that at the very least we the diagnostic should be made better.