[swift-evolution] Pitch: Member lookup on String should not find members of NSString

[Slava Pestov]

Hi,

Members of NSString, except those defined in Foundation, are available on values of type String. For example,

extension NSString {
  @objc func foo() {}
}

let s: String = “hello”

s.foo()

We don’t do this for any other bridged types, for instance NSArray methods are not imported as Array methods. It’s literally a special case in the type checker for member lookup on String.

This behavior doesn’t really much sense conceptually and it was put in as a stop-gap in Swift 1 to beef up the String API. I would like to phase it out as follows:

- Unconditional warning in Swift 4.1, with a fixit to insert an ‘as NSString’ cast
- Error in Swift 5 with -swift-version 5

What does everyone think about this?

Slava