duplicates:

• SR-8125 Swift 4.2 exhaustivity checker regression

Issue Description:

In Swift 4.1, Swift's switch exhaustiveness checker is happy to let you skip handling cases that cannot be constructed (because they involve a type with no values). Swift 4.2 has a regression where it only lets you skip the case if the type involved is Never but does not let you skip the case anymore if the type is some other no-valued type.

Reproduction:

enum MyNever {}

enum Foo<T,U> {
    case one(T)
    case two(U)
    case three
}

func foo(_ foo: Foo<Int,MyNever>) {
    switch foo {
    case .one(let x): print(x)
    // skip two, the value is Never
    case .three: print("three!")
    }
}

This compiles just fine in Swift 4.1 but fails in Swift 4.2 with the error

unnamed.swift:10:5: error: switch must be exhaustive
    switch foo {
    ^
unnamed.swift:10:5: note: add missing case: '.two(_)'
    switch foo {
    ^