Use conditional types for improved union type support about utility-types HOT 9 CLOSED

Consider this:

interface Foo {
  name: string;
  id: string;
  foo: boolean;
}

interface Bar {
  name: string;
  id: string;
  bar: number;
}

type FooBar = Omit<Foo | Bar, 'name'>;

let fooBar!: FooBar;

Without the conditional type (which wraps Pick), FooBar just picks the common part and downgrade the union type.

from utility-types.

Thanks for detailed explanation @vilic,
After some consideration that might be usefull, but it will require quite the effort as the change should be included in all relevant types to remain consistent and add new type test cases with unions input for each of the Type Operators.

I'll try to plan the milestone for it.

from utility-types.

What is it suppose to change in comparison to my current implementation? It's using conditional types already, have you checked it?

export type Omit<T, K extends keyof T> = Pick<T, SetComplement<keyof T, K>>;

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More info: https://gist.github.com/donabrams/b849927f5a0160081db913e3d52cc7b3

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I'm going to rollback the change from this proposal aiming to increase convenience for union types because it is breaking Generic Types support, which for me is more important to make it work (especially breaking in usage with Higher order components).

Consider these examples, which are all broken when Omit become distributive:

const fn = <T extends Props>(props: T) => {
    const { age, ...rest } = props;
    // @dts-jest:pass:snap
    const a: Omit<T, 'age'> = rest;
  };

const fn = <T extends Props>(props: T) => {
    const { age, ...rest } = props;
    // @dts-jest:pass:snap
    const a: Intersection<T, Omit<T, 'age'>> = rest;
  };

const fn = <T extends Props>(props: T) => {
    const { age, ...rest } = props;
    // @dts-jest:pass:snap
    const a: Overwrite<Omit<T, 'age'>, T> = rest;
  };

const fn = <T extends Props>(props: T) => {
    const { age, ...rest } = props;
    // @dts-jest:pass:snap
    const a: Assign<{}, Omit<T, 'age'>> = rest;
  };

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Also, the previous Omit was not consistent with Pick and incompatible with build-in Omit coming in TypeScript v3.5.

Now it's fixed.

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