Switch on unknown should narrow like if

[jack-williams]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: switch narrow unknown if

I realised that the most compelling use-case of narrowing unknown by === is for a selection of strings/enums, but IMO, this is cleaner when written as a switch.

Code

enum SomeResponse {
    Yes,
    No,
    Maybe,
}

function foo(x: unknown): SomeResponse {
    switch (x) {
        case SomeResponse.Yes:
        case SomeResponse.No:
        case SomeResponse.Maybe:
            return x;
        default:
            throw new Error("I don't know. Can you repeat the question?");
    }
}

Expected behavior:
No error.

Actual behavior:
Error on return statement: unknown is not assignable to SomeResponse

Playground Link: playground

Related Issues: #25720 #25172

[jack-williams]

Fix here #29711