Assigning incompatible types does not raise an error

[bagbag]

TypeScript Version: tested with 3.5, 3.6, next

Search Terms:
typescript allows assigning incompatible types
typescript allows assigning generic interfaces without full overlap

Code

type Entity = { id: string };
type Participant = Entity & { participantName: string };
type Trainer = Entity & { trainerName: string };
type Master = Trainer & { masterName: string };

interface EntityRepository<T extends Entity> {
 load<U extends T = T>(id: string): Promise<U>;
}

// should not work as Participant doesn't extend Trainer
const a: EntityRepository<Trainer> = (null as any) as EntityRepository<Participant>;

// doesn't work as expected, as Participant doesn't extend Trainer
a.load<Participant>("");

// works as expected, as Master extends Trainer
a.load<Master>("");

Expected behavior:
Assigning EntityRepository<Participant> to EntityRepository<Trainer> should raise an error, as Participant is not assignable to Trainer.

Actual behavior:
Typescript does not complain at all for the assignation.

Extra note:
If I remove U extends T = T from load and return Promise<T> instead of Promise<U> everything works as expected.
A workaround I found for myself is adding a fake property _type: T to the interface.

Playground Link:
Playground

Related Issues:
#31006

[jack-williams]

This is fixed by #31029.

[bagbag]

Oh, I didn't see the pull request in the related issue and wasn't sure whether it is the same bug.

Thanks!

[typescript-bot]

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