Class infers wrong generic parameter since e1874f3

[krryan]

🔎 Search Terms

"infer"

🕗 Version & Regression Information

• This changed in commit e1874f3

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240404#code/KYDwDg9gTgLgBAYwDYEMDOa4BVhpgHgEEAaOAIQD44BvAKDgcQgDs8oBXBGaOACjHYAjJAEsEcFAC44JOAOFi4g6WQCUNAL616jGLgIBhCrwTSDq6Tjz4ypIzR2NGUYDHZRmcZsADu2fbwwABYiaAB0gqQIqgDcjnBaWkA

💻 Code

export class Test<A, B> {
    constructor (public a: A, public b: B) {}

    test<C>(c: C): Test<B, C> {
        return new Test(this.b, c);
    }
}

🙁 Actual behavior

The invocation of new Test infers generic parameters of <C, C>, even though this.b is B and c is C, which should pretty clearly result in <B, C>. This incorrect inference causes an error on return since it does not match the return type annotation:

Type 'Test<C, C>' is not assignable to type 'Test<B, C>'.
  Type 'C' is not assignable to type 'B'.
    'B' could be instantiated with an arbitrary type which could be unrelated to 'C'.(2322)

Explicitly invoking new Test<B, C> compiles without error.

🙂 Expected behavior

Compiles successfully.

Additional information about the issue

I initially posted this with a slightly more complex example, where the second argument to both Test’s constructor and the test method was a callback, (v: A) => B and (v: B) => C, respectively. I realized that this callback was a bit of a red herring and the bug occurs even when these are simple, explicitly-annotated values.

[Andarist]

Extra repro for what I believe to be the same problem (cc @mikearnaldi):

interface Supervisor<out T> {
  zip<A>(right: Supervisor<A>): Supervisor<[T, A]>;
}

export class Zip<out T0, out T1> implements Supervisor<readonly [T0, T1]> {
  constructor(
    readonly left: Supervisor<T0>,
    readonly right: Supervisor<T1>,
  ) {}

  zip<A>(right: Supervisor<A>): Supervisor<[[T0, T1], A]> {
    return new Zip(this, right);
  }
}

[weswigham]

#58168 is up with a fix - just need to expand a scope we're looking at from the constructor signature to the whole class body.