Key Remapping and keyof modifier

[shauns]

TypeScript Version: 4.1.0-beta

Using the new key remapping feature, with a remapped object with never keys (for omitting). Passing the remapped object to keyof returns never, instead of the keys from the remapped object. Checks with the remapped object itself work as expected - its only when then using keyof do they fail.

Search Terms:

• key remapping
• keyof with never

Code

type Example = {foo: string, bar: number}
  
type PickByValueType<T, U> = {
  [K in keyof T as T[K] extends U ? K : never]: T[K]
}

type T1 = PickByValueType<Example, string>
const e1: T1 = {
    foo: "hello"
}
type T2 = keyof T1
const e2: T2 = "foo"

Expected behavior:

keyof T1 should be "foo"

Actual behavior:

keyof T1 is never

T1 correctly picks out the expected keys based on the condition check, but when ran through keyof it seems to lose the "set" keys.

Playground Link:

https://www.typescriptlang.org/play?ts=4.1.0-pr-40336-88&ssl=4&ssc=41&pln=4&pc=46#code/C4TwDgpgBAogHgQwLZgDbQLxQN4DMD2+AXFAM7ABOAlgHYDmANFAEYIUk0CuSzEFAvgCgoUQaEhQAClQDGAawBCIAGoJUnCABVwEADyamAVQB8ULNmFQA2gGkotKHIgh8uKJqgJS72wF0oEHDAEDQAJt6GUAD8UHYcEABufL4kmn6CQmI67gCMZlKyiipqGtqQuvDIaBBM5NT0xoIy+DTkATmpeeaWIgTEUABEABYQqKj4AxlZEpoATPlOLm6aOU0tbRCzqfNYA32TQA

Related Issues:

• Original PR: Template literal types and mapped type 'as' clauses #40336

[ahejlsberg]

Ah, the issue here is that the conditional type in the as clause is not distributive, so when applied to a union type (which we do in getIndexType) it isn't applied to each key individually. A workaround is this:

type PickByValueType<T, U> = {
  [K in keyof T as K extends K ? T[K] extends U ? K : never : never]: T[K]
}

But of course it should work without the explicit distribution trick.

[shauns]

Thanks for sharing the workaround @ahejlsberg ! I see there's an open PR for a fix, so I hope the bug report was useful. And thanks for the addition of the feature -- I've already two scenarios where it's proved useful.