Using keyof with key remapping to exclude index signatures doesn't return known keys #41966
Description
TypeScript Version: 4.2.0-dev.20201211
Search Terms:
key remapping
remap index signature
keyof returns number
Code
type Compute<A> = { [K in keyof A]: Compute<A[K]>; } & {};
type EqualsTest<T> = <A>() => A extends T ? 1 : 0;
type Equals<A1, A2> = EqualsTest<A2> extends EqualsTest<A1> ? 1 : 0;
type Filter<K, I> = Equals<K, I> extends 1 ? never : K;
type OmitIndex<T, I extends string | number> = {
[K in keyof T as Filter<K, I>]: T[K];
};
type IndexObject = { [key: string]: unknown; };
type FooBar = { foo: "hello"; bar: "world"; };
type WithIndex = Compute<FooBar & IndexObject>; // { [x: string]: {}; foo: "hello"; bar: "world"; } <-- OK
type WithoutIndex = OmitIndex<WithIndex, string>; // { foo: "hello"; bar: "world"; } <-- OK
type FooBarKey = keyof FooBar; // "foo" | "bar" <-- OK
type WithIndexKey = keyof WithIndex; // string | number <-- Expected: string
type WithoutIndexKey = keyof WithoutIndex; // number <-- Expected: "foo" | "bar"Expected behavior:
Using keyof with the above remapped type (WithoutIndex) should return the known keys (i.e. "foo" | "bar").
Actual behavior:
While the key remapping in OmitIndex appears to successfully omit the string index signature (e.g. OmitIndex<WithIndex, string> --> { foo: "hello"; bar: "world"; }); using keyof with the resulting type returns number.
Playground Link:
Related Issues: