Omit breaks in 3.9.2

[zombieJ]

TypeScript Version:
3.9.2

Search Terms:
3.9.2 Omit

Code

interface A { 
  my: 'aaa';
}

interface B { 
  my: 'bbb';
}

interface C extends Omit<A & B, 'my'> { 
  my: string;
}

Expected behavior:

Can define my with Omited like prev version.

Actual behavior:

Property 'my' of type 'string' is not assignable to string index type 'never'.(2411)

Playground Link:

https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=12&pc=1#code/JYOwLgpgTgZghgYwgAgILIN7ILAChnIC2AngFzIDkc1FA3HgL556iSyIoBCmO+RZlAEbC6jZrlbR4SZAGFkEAB6QQAEwDOyAPKFgYADzoAZMk4AaSiQoA+HngIly6sFFABzeria4gA

Related Issues:

[kitsonk]

Effectively duplicate of #38241, related to breaking change #37195.

[ahejlsberg]

The issue here is that A & B is an empty intersection (i.e. a type for which you couldn't possibly construct a value) that now reduces to never. We previously didn't catch such types.

You could instead write your code as

type Base = Omit<A, 'my'> & Omit<B, 'my'>;

interface C extends Base { 
  my: string;
}

That is, omit the conflicting discriminants before you intersect the types.

[zombieJ]

@ahejlsberg , but never can be omit:

interface A { 
  my: never;
}

interface C extends Omit<A, 'my'> { 
  my: string;
}

https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=8&pc=1#code/JYOwLgpgTgZghgYwgAgILIN7ILAChnIC2AngFzIgQBu0A3HgL556iSyIoDCyEAHpCAAmAZ2QB5QsDAAeVABpkAchKKAfJhz4iZZMLBRQAc3q4muIA

[ahejlsberg]

Yes, but any type that includes a property of type never is a type with no possible values, so we're not talking about something that couldn't exist in the real world. Ideally we'd reduce all such types to never (that is, instead of { my: never } we'd simply have never) but it is costly to check for and tricky to do in a lazily evaluated type system.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.