Generic constraint generates a TS2344

TypeScript Version: 3.7.x, 3.8.1, 3.9.x (next)

Search Terms: Generic constraint 2344

Code

type KeyOf<T> = keyof ((T extends any ? (k: T)=>void : never) extends ((k: infer I)=>void) ? I : never);

type Constraint<T> = KeyOf<T> extends never
  ? any
    // : KeyOf<any> extends KeyOf<T>
    // ? any
  : (
    'a' extends KeyOf<T>
      ? never
      : any
    );

type ForceConstraint<T> = T extends Constraint<T> ? T : never;

interface Interface<T extends Constraint<T>> {
  b: string;
}

type Infer<CB> = CB extends () => infer TReturn
  ? ForceConstraint<TReturn>
  : never;

type InferWithConstraint<CB> = ForceConstraint<Infer<CB>>;

type InferInterfaceFromCallback<CB> = Interface<InferWithConstraint<CB>>; // invalid => NOT OK
type InferInterfaceFromCallback1<CB> = Interface<Infer<CB>;> // invalid too => NOT OK

// just debug purpose
type A1 = Interface<unknown>;
type A2 = Interface<any>; // invalid => OK
type A3 = Interface<never>;
type A4 = Interface<undefined>;
type A5 = Interface<number>;
type A6 = Interface<string>;
type A7 = Interface<null>;
type A8 = Interface<object>;
type A9 = Interface<{}>;
type A_FAIL1 = Interface<{ a: 'b' }>; // invalid => OK
type A_FAIL2 = Interface<{ a: 'b' } | 1>; // invalid => OK

Playground Link

Expected behavior:

type InferInterfaceFromCallback<CB> = Interface<InferWithConstraint<CB>>;

Should be valid

Explanation:

So, i was trying to apply a specific constraint on a generic type.

Constraint<T> 'returns' never if T contains a property called a, else it returns any

ForceConstraint<T> tries to help typescript to know if T is valid, else it returns never

Interface<T extends Constraint<T>> forces its generic type to respect Constraint

Infer<CB> infers the return type of CB and ensures it follows Constraint

InferWithConstraint<CB> forces again Infer<CB> to be compliant with Constraint (even if it is already)

So Interface<InferWithConstraint<CB>> should be valid, sadly, typescript reports:

TS2344: Type 'ForceConstraint<Infer<CB>>' does not satisfy the constraint 'Constraint<ForceConstraint<Infer<CB>>>'.

Potentially related issues:

• https://github.com/microsoft/TypeScript/issues/31736
• https://github.com/microsoft/TypeScript/issues/34604

Commencez par le Playground Link fourni et reproduisez le diagnostic TS2344 à l’aide de l’exemple minimal de type générique. Comparez la validité attendue de Interface<InferWithConstraint> avec l’erreur de contrainte signalée et examinez les Issues associées 31736 et 34604. Le travail est terminé lorsque l’alias de type valide ne produit plus TS2344, tandis que les exemples invalides restent rejetés.