Bad error message for type used as index

TypeScript Version: 3.1.0-dev.20180717

Code

type S = "a" | "b";
type O = { [S]: number };

Expected behavior:

Error message telling me to use K in S.

Actual behavior:

src/a.ts:2:13 - error TS2693: 'S' only refers to a type, but is being used as a value here.

[a-tarasyuk]

@mhegazy @Andy-MS @DanielRosenwasser which message will be acceptable to this case?

I'm thinking something like:
'S' is a type, not a symbol. To use 'S' as a key type of a mapped type, use 'K in S'.

(Note that with const S = Symbol(); the code would compile as-is.)
Note that a mapped type will only work here if the type does not have any other properties; see #24286.
We could also add a codefix triggered by this diagnostic.

[DanielRosenwasser]

Thanks @a-tarasyuk!