Omit is broken / Modifiers not inferred

[millsp]

@DanielRosenwasser

The new version of Omit discards the field modifiers. The solution is to avoid using a type directly in a keyof statement as TS loses the inference of the field modifiers (optional, readonly).

I am not sure if it should behave this way, as you looked confident writing the Omit type, it's probably part of another problem. In short, TS discards the inference of field modifiers on mapped types that map with another type (instead of keyof).

TypeScript Version: 3.5.0-dev.20190501

Search Terms:
mapped type readonly lose modifiers

Code

type Omit<T, K extends keyof any> = {
    [P in Exclude<keyof T, K>]: T[P]
} //     ^^^^^^^^^^^^^^^^^^^^ optional & readonly not inferred

// Mapping with a type like Exclude<keyof T, K> caused TS to forget modifiers

type O = {
    readonly a: string
    b?: number
    c?: any
}

type test00 = Omit<O, 'b'>

type WhatWeGet = {
    a: string;
    c: any;
} // === test00

type WhatItShouldBe = {
    readonly a: string;
    c?: any;
} // !== test00

Expected behavior:
Should preserve the modifiers

Actual behavior:
Loses the modifiers on the fields

Playground Link:

Related Issues:

[jcalz]

Duplicate of #31190