Unexpected "An index signature parameter type must be 'string' or 'number'." error with string union type.

[Strate]

TypeScript Version:

1.8.10

Code

type StringEnum = "one" | "two"

interface IndexedObject {
  [index: StringEnum]: boolean
}

Expected behavior:
No error

Actual behavior:
error TS1023: An index signature parameter type must be 'string' or 'number'.

[evansb]

Reproducible in latest master.

At the moment only string or number keyword can be used as index parameter type, so a simpler type alias below also fails.

type StringEnum = string;

interface IndexedObject {
  [index: StringEnum]: boolean
}

Edit: This is intended behaviour, see the grammar of Index Signatures

[Strate]

This is intended behaviour, see the grammar of Index Signatures

I think that Index Signatures could be extended without regression

[sandersn]

Duplicate of #8336, #5683, #2491, and #7656. Some of these have good discussion. This is a popular request.