InstanceType and constructor return type inference doesn’t work for built‑in subtypes of ErrorConstructor

[ExE-Boss]

🔎 Search Terms

• "ErrorConstructor"
• "InstanceType Error"
• "InstanceType ErrorConstructor"

🕗 Version & Regression Information

• This is the behaviour in every version I tried, and I reviewed the FAQ for entries about this.
• I was unable to test this on prior versions because dtslint only tests back to TypeScript 5.0, and the bug workbench failed to download versions older than TypeScript 4.0.

⏯ Playground Link

https://www.typescriptlang.org/dev/bug-workbench/?#code/CYUwxgNghgTiAEkoGdnwMIFdkBcD2AtgKIwx4zwgAeOIAdsGiWRQN4BQAvu+zgJ4AHBADkiARngBeeAEk6uKHTAgAKoJAAeIgDcoEZuXR55OGJjD4YAPgDc7APT34z+AD0A-D35D4ogExSsiaKympCGgBKigDmIAYwRiZmFuS2Dk4uHl7qvkQAzIFyCkqq6pEgAGYgcCXxibjJlmmOLm6evDmiACyFwSVhmgDKfHQ4UFR1xg3mTXYtme3eIkQArL3FoWUDk0kzqXMZzlkdPqIAbOtj-WUAqhEyO9Mp1getx0sYRJchpeFLeBUMNh8MRSPt0m9PEA

💻 Code

declare class CustomError extends Error {
}

type NE1 = InstanceType<EvalErrorConstructor>;
//    ^?

type NE2 = InstanceType<RangeErrorConstructor>;
//    ^?

type NE3 = InstanceType<ReferenceErrorConstructor>;
//    ^?

type NE4 = InstanceType<SyntaxErrorConstructor>;
//    ^?

type NE5 = InstanceType<TypeErrorConstructor>;
//    ^?

type NE6 = InstanceType<URIErrorConstructor>;
//    ^?

type CE = InstanceType<typeof CustomError>;
//    ^?

Workbench Repro

🙁 Actual behavior

Type query results:

type NE1 = Error
type NE2 = Error
type NE3 = Error
type NE4 = Error
type NE5 = Error
type NE6 = Error
type CE = CustomError

🙂 Expected behavior

Type query results:

type NE1 = EvalError
type NE2 = RangeError
type NE3 = ReferenceError
type NE4 = SyntaxError
type NE5 = TypeError
type NE6 = URIError
type CE = CustomError

Additional information about the issue

Same happens with:

declare function newError<E extends Error>(ctor: new (msg?: string) => E): E;
declare class CustomError extends Error {
}

let ne1 = newError(EvalError);
//   ^?

let ne2 = newError(RangeError);
//   ^?

let ne3 = newError(ReferenceError);
//   ^?

let ne4 = newError(SyntaxError);
//   ^?

let ne5 = newError(TypeError);
//   ^?

let ne6 = newError(URIError);
//   ^?

let ce = newError(CustomError);
//   ^?

Workbench Repro

Related issues

• @types/babel__traverse: align type definitions closer to Babel source DefinitelyTyped/DefinitelyTyped#64628 (review)

[mkantor]

Here's a generalized example of the relevant type system behavior.

The difference between the built-in error constructor types and your custom one is that the built-in ones inherit from ErrorConstructor on their static side.

These types are all structurally-identical though so arguably none of this behavior is "wrong" (but it does lead to non-ideal type info).

[ExE-Boss]

Changing the ErrorConstructor subtypes to use:

type OmitCallSignatures<T> = { [K in keyof T]: T[K] };

interface TypeErrorConstructor extends OmitCallSignatures<ErrorConstructor> {
}

would fix this specific issue, as the static side also inherits the call signatures by default.

I would expect that subinterface new/⁠call signatures would take priority over superinterface new/⁠call signatures in ReturnType<T>/InstanceType<T>, function newError<T>(…), and other new? (...args: infer A) => infer R types.

[RyanCavanaugh]

This is a weird interaction between overload resolution and ReturnType and I'm kind of surprised this is the first time anyone's noticed.

If you have two interfaces

interface Base {
  (): string;
}
interface Derived extends Base {
  (): "derived";
}

then we merge the overloads with the derived class first:

(): "derived";
(): string;

This is desirable because in a call position f(), the first match signature of f is the one we pick, and when f is Derived you obviously want "derived"

ReturnType does the opposite; it picks the last overload on the assumption that you have a signature list like

(s: string): number;
(s: number): string;
(s: string | number): string | number;

where the last overload is the "catch-all" that has the widest return type.

I think the right fix would be to split up ErrorConstructor so that its call/construct signatures aren't part of the interface that gets extended, e.g.

interface ErrorConstructorStatics {
  // whatever goes here
}
interface ErrorConstructor extends ErrorConstructor {
  new(message?: string): Error;
}

interface SpecialErrorConstructor extends ErrorConstructorStatics {
  new(): SpecialError;
}

this would break existing interface merges on ErrorConstructor - they'd have to merge the Statics one instead - but preserves all other useful semantics. Seems like an OK change

[ExE-Boss]

One option would be to filter out inherited overloads which are wholly subsumed by overloads declared on the subinterface, e.g.: for:

interface ErrorConstructor extends ErrorConstructor {
	new (message?: string): Error;
	new (message?: string, options?: ErrorOptions): Error;
}

interface SpecialErrorConstructor extends ErrorConstructor {
	new (message?: string): SpecialError;
	new (message?: string, options?: ErrorOptions): SpecialError;
}

Because the new signatures of SpecialErrorConstructor wholly shadows the inherited new signatures from ErrorConstructor, it’d be filtered out when doing overload resolution for ReturnType/InstanceType, e.g.:

Instead of getting a list of overloads for SpecialErrorConstructor (where the last two are unreachable):

• new (message?: string): SpecialError;
• new (message?: string, options?: ErrorOptions): SpecialError;
• new (message?: string): Error;
• new (message?: string, options?: ErrorOptions): Error;

The list of overloads checked by ReturnType/InstanceType would simply be:

• new (message?: string): SpecialError;
• new (message?: string, options?: ErrorOptions): SpecialError;