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

Beginne mit dem bereitgestellten Playground Link und reproduziere die TS2344-Diagnose anhand des minimalen Generik-Typbeispiels. Vergleiche die erwartete Gültigkeit von Interface<InferWithConstraint> mit dem gemeldeten Constraint-Fehler und sieh dir die verwandten Issues 31736 und 34604 an. Als erledigt gilt die Aufgabe, wenn der gültige Typalias nicht mehr TS2344 erzeugt, während die ungültigen Beispiele weiterhin abgewiesen werden.