Switch statement does not flag non-comparable cases with non-overlapping literal and non-literal union types

[ericbf]

TypeScript Version: 3.0.0-dev.201xxxxx

Search Terms:
Switch, non-comparable types, false negative

Code

declare let message: number | "hello"

switch (message) {
    case "hi":
        break
}

Expected behavior:
Type '"hi"' is not comparable to type 'number | "hello"'.
(2678)

Actual behavior:
No error.

Playground Link: https://www.typescriptlang.org/play/?ssl=1&ssc=31&pln=1&pc=22#code/CYUwxgNghgTiAEEQBd4FsQGdNQOYgC54A7AVzQCMQZ4AfeAIgAsQIIB7BgKC8wHcAlsjBN4ACgzY8IAJTwA3l3jL4YKJgTMBDAkpX6KcKAGsuAXyA

Related Issues: None that I found

[hoangpham95]

Can I take a stab at this issue?

[ericbf]

Please!

[ericbf]

This is still an issue on at least 4.8.4:

https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEEQBd4FsQGdNQOYgC54A7AVzQCMQZ4AfeAIgAsQIIB7BgKC8wHcAlsjBN4ACgzY8IAJTwA3lwCQYKJgTMBDAsqUU4UANZcAvkA

[ericbf]

I opened a PR that fixes this issue. May need some assistance with ascertaining any unexpected side effects.