Template Literal Types derived from the type keys cannot be used as Indexed Access Types in generic contexts about typescript HOT 3 OPEN

What's the use of being able to write this, if it were allowed?

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I came across this issue when writing stricter types for Object.keys(...) and Object.values(...).
They are

type KeysOf<T extends object> = `${keyof T & (string | number)}`; // Simplified.

and

type ValuesOf<T extends object> = T[KeysOf<T>]; // Simplified.

respectively.

The issue is that KeysOf<T> cannot be used to index type T even though the type is guaranteed to contain only valid keys, which could be algorithmically deduced from the definition.

from typescript.

The fundamental issue here is consistency. For any concrete type X, if another type Y can be used to index it as X[Y], then the expression X[`${Y}`] (if valid) always gives the same resulting type as X[Y].

This bug report / suggestion is about keeping this behaviour consistent when extending the same rule to generic types.
Since there are no types that present an exception to the rule, it should work with generics as well.

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