TS2536: Type 'unique symbol' cannot be used to index type 'T'.

[yortus]

TypeScript Version: 2.8.0-dev.20180211

Search Terms:

• "TS2536"
• "Type 'unique symbol' cannot be used"

Code

const STR = 'a string literal';
const SYM = Symbol('a unique symbol');

const value = {
    [STR]: 'foo',
    [SYM]: 42,
};

type TypeOfStr<T extends { [STR]: any; }> = T[typeof STR];  // OK
type TypeOfSym<T extends { [SYM]: any; }> = T[typeof SYM];  // ERROR
                                        //  ^^^^^^^^^^^
                                        //  Type 'unique symbol' cannot be used to index type 'T'

type T1 = TypeOfStr<typeof value>;  // T1 is string
type T2 = TypeOfSym<typeof value>;  // T2 is number

Expected behavior:
No errors. If it works for string literal types, it seems it should also work for unique symbol types.

Actual behavior:
TS2536 error with unique symbol as shown above. But the type is still correctly inferred on the last line.

[mhegazy]

Should be fixed by #22339