StringProtocol.contains(_:) returns false for empty string

[yo1995]

Description

Please see the Swift forum post for details: https://forums.swift.org/t/nsstring-contains-returns-false-for-empty-string-argument/3949

It seems that the current implementation in Foundation has this problem, while the implementation in _StringProcessing doesn't.

// Without importing Foundation
"foo".contains("") // ⟹ true

import Foundation
"foo".contains("") // ⟹ false

[itingliu]

We're aware of this discrepancy, but this is one of those behavior we need to maintain for compatibility reasons.