[SR-16011] Undefined behavior when switching over a frozen enum imported from C

[grynspan]

Previous ID	SR-16011
Radar	None
Original Reporter	@grynspan
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	swift
Labels	Bug
Assignee	None
Priority	Medium

md5: 7e6f700cc048d77f075128ff58486486

Issue Description:

Given the following code:

import Foundation

func ub(_ i: Int) {
    guard let res = ComparisonResult(rawValue: i) else {
        return
    }
    switch res {
    case .orderedAscending, .orderedDescending, .orderedSame:
        print("No undefined behavior here.")
    }
}
ub(2)

The program will crash with the message:

> Fatal error: unexpected enum case 'NSComparisonResult(rawValue: 2)'

At the language level, Swift is assuming that since the `switch` is exhaustive, no `default` case is needed. However, because the enum is imported from C, it might have cases that were not visible to the Swift importer at import time, so `init(rawValue🙂` always produces an instance of `ComparisonResult` even if the input value is invalid, despite it being frozen. The issue is not specific to `ComparisonResult`, but it is an easy way to demonstrate it.

The language should probably change the synthesized initializer for an imported frozen enum such that it returns `nil` for cases that are not visible at import time.