No index signature when a generic type has a symbol property

[Conaclos]

Search Terms: "symbol", "no index signature", "symbol enumerable"

Tested TypeScript versions: 2.7.2 and 2.8.0-dev.20180302

Code

declare const SYM: unique symbol
type DecoratedPartial <T> = Partial<T> & {
    [SYM]: Function
}
function traverse <T> (ref: DecoratedPartial<T>): void {
    for (const p in ref) {
        const v = ref[p as keyof DecoratedPartial<T>] // error, see below
    }
    const f = ref[SYM]
}

Expected behavior:

No error when noImplicitAny is enabled.

Actual behavior:

The following error is reported:

error TS7017: Element implicitly has an 'any' type
because type 'DecoratedPartial<T>' has no index signature.

Note that this error only occurs when noImplicitAny is turned on.

Playground Link: play with it. Don't forget to enable noImplicitAny.

Related Issues:
The following issue could be related to:

• TS2536: Type 'unique symbol' cannot be used to index type 'T'. #21912
• Issues indexing with unique symbols on generic types #22320

Workaround:

Write an interface whithout the symbol properties (here Partial), and use this interface for traversing (using type assertions).

...
const v = (ref as Partial<T>)[p as keyof Partial<T>]
...

[mhegazy]

should be fixed in latest.

[mhegazy]

fixed by #22339