Instanceof Type Inference Doesn't Take Generic Defaults Into Account

[joeedh]

🔎 Search Terms

inference, generic, parameter, default

🕗 Version & Regression Information

Behavior is the same in all version I tried.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.2#code/PQKgBApgzgNglgOwC4FoAmcoEMBGMJgjABQxEAHgA4D2ATkmEgJ6UECiAbhMgJIIAWEWnCQARCADMsAVxgMAvGADejFhABcYBBC60wAXwDcZKnQbNWYTtyR9BwpABU1YRSosaw0hAGsE1AHcEA2NiDzAAcQgka2QoAB5YpChIciRuNBSkuyERZ1YAPlcrLjiAbQAiDwqAXVT0hEywKCRhBABzYjAwAH4SmyhK6pqusE1tXWMKGnpVSySomNLkxPqMrOWch3yIIsVFpIS2AqnTWfCtkUP4xzXGlJa29oAaKzum7IFcpzU95VGPJpbgAfSLRa7HYxGUjTMxgADGMCwUBSACFkRBEssUhQGk1Hoh2sUCR0ikpRhAALZXZYACh0Nk0hwAlMp9MR2SYZgxEciUgBBeKjQ7vDY2S4-SyKT72ETiKSyJDEIq49ZgdFQTELcHY+IS64VajaCpgUFVALUCqvQ4FW1smFnHlIlFgADCWIGov6vC+2xc0s2vrlkhkchVaTVgtG+t1VX4tAgEBNZok1GktCt3uSBWV9uI8KNLQRxW0ATdtOZxgLCCLOAxmg1BEU8NC8IAdFSaTZaXGE0nK8Q4BIwLS65qwIgWlgEPCINRh67WeTusBgM1+GmYGgwDgCNPILRaHRnqNV3MCIgJEJuLOwFI4Pht0hqF5x+1uLl4WBKFhaFhKdEQinmuaAhoqhjnmA847hiE4pO605MDm3RjhAHbUks3a9v2oQrmu0zwPCIgIsiDBoNQ0AkdI477kIR60KM1ZFvCpHFKhYDIm6xjdCxLToV2yA9kg8Y4RyxBAA

💻 Code

/* eslint-disable */

// The idea here is to propagate a primitive string union of event types
// down a class hierarchy.  The weird boilerplate is to keep TS 
// from automatically inferring to `string`.

export type EventInheritDefault = { type: never };
export type EventInheritType = { type: unknown };

type GetEvents<Events extends EventInheritType> = Events["type"] extends string
  ? Events["type"]
  : never;
export type EventGetEvents<E extends EventInheritType> = GetEvents<E>;
export type InheritEvents<T extends string, E extends EventInheritType> = {
  type: T | GetEvents<E>;
};

export class Base<Events extends string = string> {
  emitEvent(event: Events) {}
}

export class A<
  Events extends EventInheritType = EventInheritDefault
> extends Base<EventGetEvents<InheritEvents<"one" | "two", Events>>> {}

export class C<Events extends EventInheritType = EventInheritDefault> extends A<
  InheritEvents<"three" | "four", Events>
> {}

const c = new C();
const base: Base = c;

c.emitEvent("three");
if (base instanceof C) {
  // should be an error
  base.emitEvent("thre");

  // explicit cast does cause an error
  const cast = base as C;
  cast.emitEvent("thre");
}

🙁 Actual behavior

The instanceof check resolved to C<any> instead of using the generic parameter's default.

🙂 Expected behavior

The instanceof check should not resolve to C<any> and should produce an error when calling base.emitEvent("thre")

Additional information about the issue

It seems odd that an instanceof check would produce a different result then a cast. The following code actually does work correctly:

const C2 = C as {new(): C}
if (base instanceof C2) {
  // this now errors as expected
  base.emitEvent('thre')
}

[jcalz]

Duplicate #48438

[typescript-bot]

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