Nonsensical error message

[ahejlsberg]

TypeScript Version: 2.9.0-dev

Code

type Wat<T extends string> = { [x: T]: string };  // Error: An index signature cannot be a union type ?!?

Expected behavior:

Meaningful error message.

Actual behavior:

Error message complains about a union type, but there is no union type.

Related Issues:

The error message is caused by #20726. There is no verification that the type is actually a union type.

[mhegazy]

@kujon wanna take a crack at this one as well?