`ObjectOmit` does not carry across properties from all variants in union

[OliverJAsh]

Is this expected behaviour? Here are two examples using ObjectDiff and ObjectOmit.

import { ObjectDiff, ObjectOmit } from 'typelevel-ts/lib';

{
  type Common = {
    commonProp: string;
  };

  type Full = Common &
    (
      | {
          type: 'foo';
          foo: string;
        }
      | { type: 'bar'; bar: string });
  type Defaults = Pick<Full, 'commonProp'>;

  type Input = ObjectDiff<Full, Defaults>;

  const check = (i: Input) => i;
  check({
    type: 'foo',
    foo: 'foo', // unexpected error
  });
  check({
    type: 'bar',
    bar: 'bar', // unexpected error
  });
}

{
  type Common = {
    commonProp: string;
  };

  type Full = Common &
    (
      | {
          type: 'foo';
          foo: string;
        }
      | { type: 'bar'; bar: string });
  
  type Input = ObjectOmit<Full, 'commonProp'>;

  const check = (i: Input) => i;
  check({
    type: 'foo',
    foo: 'foo', // unexpected error
  });
  check({
    type: 'bar',
    bar: 'bar', // unexpected error
  });
}

[OliverJAsh]

Seems like the crux of the issue here is keyof:

type Foo = keyof Full;
// expected "commonProp" | "type" | "bar" | "foo", got "commonProp" | "type"

[gcanti]

yes, keyof a union yields the common fields

[OliverJAsh]

Not an issue with this library in that case. Thanks!

To your knowledge, is there any way to omit common properties from a union, and retain the union members?

[OliverJAsh]

I believe we can fix this now with conditional types:

type Omit<T, K extends keyof T> = T extends any ? Pick<T, Exclude<keyof T, K>> : never

microsoft/TypeScript#12215 (comment)