TS2345 not thrown when calling a function whose parameter is a lookup type of a generic type

[jiakaiz-g]

TypeScript Version: 3.9.2

Search Terms:

TS2345
generic type
lookup type

Code

interface AEvent {
  foo: number;
}

interface BEvent extends AEvent {
  bar: number;
}

export declare interface AEventMap {
  'event': AEvent;
}

export declare interface BEventMap extends AEventMap {
  'event': BEvent;
}

class A<EventMap extends AEventMap = AEventMap> {
  listeners: Array<(e: EventMap['event']) => void> = [];

  addListener(listener: (e: EventMap['event']) => void) {
    this.listeners.push(listener);
  }

  triggerEvent(e: EventMap['event']) {
    for (const listener of this.listeners) {
      listener(e);
    }
  }

  run() {
    this.triggerEvent({ foo: 1 });  // Should throw TS2345.
  }
}

class B extends A<BEventMap> {}

const b = new B();
b.addListener((e) => console.log(e.bar));  // 'bar' may not exist.

b.run();

Expected behavior:

TS2345 should be thrown at the this.triggerEvent({ foo: 1 }); statement because EventMap['event'] could be a subtype of AEvent, in which case { foo: 1 } cannot assign to it.

A simplified version of the code without lookup types gets an error correctly.
https://www.typescriptlang.org/play/#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-5EIA

Actual behavior:

No compilation error.

Playground Link:

https://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgIIFEBuFzIN4CwAUMsjAPbkBcyIArgLYBG0A3MQL7HGiSyIoAQlhxhkEAB6QQAEwDOaEbkIlkTOFBr1mbTtyKSADuShiZEBABsNKXtHhJF2cAFk4h-MVIByCM7DeNBj+7ERcRMRGJmYW1lC24PYCyML+bh6S0vJOoumeqr7+gSlKYKHhxFZwcgqoADyleZk42cG57sgAvDmu7gB8+aSWwHLS0HJBUFBwAJ51ABQQNI3uANqFot4AugCUXQOY5MAyA92rW6FeyHAyMgAyI2NQ88OjONA0i8tpaxvg23tOgcjjI9ipSKQwAALEYAOleTzksMMdDkUJej3eUB2oVIFVUYCgwAA5sToKUvsgVoZ1n5NrtBhCKFBkPMEOQQKNkAischyDBkNC4TyQOMwVcIdzMaLnhAcRLkOE8fpSFA6CB5uLVJCYUjCSSyVAKXgyJQaABGRU40gAehtyAAylDyHRLDJBVCoOQAO7IAAqDoATABmAAsAFZYVdwviqjUUuIpC1anVUu1DAM8LGOVymF1aBBfYJNaEmLCbvdpdB5otAQN2ZzyJYIPDyMTFrD1Njrcg7chvF3vMgGLNaOQxJJHlGIkQy2qNfKiEA
Related Issues:

I could not find.

[jack-williams]

Here is a good explanation of the behaviour: #27470 (comment)

[jiakaiz-g]

Thank you for the explanation!