[SR-12063] Swift should warn for switch case '.none' vs '.none?' ambiguity

[keith]

Previous ID	SR-12063
Radar	None
Original Reporter	@keith
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, TypeChecker
Assignee	@theblixguy
Priority	Medium

md5: 17a3bec14e602d0feaaa8c2f85e8aae7

Issue Description:

With this code:

enum Foo {
    case bar
    case none
}

let a: Foo? = Foo.none

switch a {
case .none: // Should be '.none?'
    print(".none")
case .bar:
    print(".bar")
default:
    print("default")
}

Everything compiles, but you may have surprising behavior where the `default` case is matched instead of `case .none`.

Following by the example of the great warning you get when you have `let a: Foo? = .none`, it would be great if there was a similar warning for the "ambiguity" here

[keith]

Looks like this concern was brought up on the other PR allowing the removal of the `?` in the first place #22486 (comment)

[theblixguy]

Good idea, I’ll take a look tomorrow! I think the fix-it should be to change it to 'nil' if you want to match against Optional.none. This is what the official documentation does as well (see https://docs.swift.org/swift-book/ReferenceManual/Patterns.html#ID424).

[theblixguy]

Fixed by #29356 Please verify using the next available snapshot.

[keith]

Thanks!