Weak generic constraint check

[awerlogus]

TypeScript Version: 4.2.0-dev.20201222

Search Terms: generic constraint check

Code

type Emit<E extends string> = { emit<T extends E>(event: T): void }

type EmitAB = Emit<'a' | 'b'>
type EmitCD = Emit<'c' | 'd'>

declare const emitAB: EmitAB

// Actually: no error
// Expected: EmitAB is not assignable to EmitCD
const emitCD: EmitCD = emitAB

Expected behavior:
EmitAB is not assignable to EmitCD

Actual behavior:
No error

[andrewbranch]

Note that neither the method nor the union types are necessary to repro:

type Emit<E extends number> = <T extends E>(event: T) => T

declare let emit0: Emit<0>;
declare let emit1: Emit<1>;

emit0 = emit1;
emit1 = emit0;

[DanielRosenwasser]

I think that this is a duplicate of #31006 - @ahejlsberg can you confirm?

[DanielRosenwasser]

Relevant design meeting might be #31132.

[awerlogus]

It's hard to imagine that accessing some type in type level closure may cause so many problems

[DanielRosenwasser]

I'm going to assume this is a duplicate.

[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.