strictBindCallApply error mentions `this` type mismatch, but elaboration points to parameters, even when `this` type matches

🔎 Search Terms

strictBindCallApply this arg error

🕗 Version & Regression Information

• This changed between versions 4.4 and 4.5

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.5.5#code/KYDwDg9gTgLgBDAnmYcDCBDANlgRhgYwGsBBOAXjgAoMoBzARgC44BnGKASwDs6AaOLToAmFtwCuAW1zAoASgoA+OBOmyA3AChQkWAmSpMOfMQBCFakOZsOPOgvLLVMqFs08YsgGaFUAMXFuAhhOCG5WOABvTThYwRYaemt2Ll4BAmw8QiIWIyziEgcnKRctOLhcBKsWFLt0zJMc9Abs0yKVEo1NAF9NTQATYAIsWlQCMPY4L0Dg0PCWAKCQibcdaHhppbm4AihgDE8AHgAVOFBPbn6IomBECC84RdmJxSobxBZjuQWZ5fCAbWOAF0ojE4nsYOIoNxLEkarY0jtKs1jNkyAAfFH5UjtTbPAHvIEAOgyOCogUGXh4wH6Ais6Vwch6fUGw1GcCwwHgGG4iCePy2E3+73uj1+c1YQJZQxGex2E3gBGReUaGKxqrcfR5fJmJMy5MuwCp3BpAgARGaGUy+kA

💻 Code

export type CallbackA = (arg1: string, arg2: number) => number;
export type CallbackB = (arg1: string) => number;

interface Functions {
    a: (arg1: string, callback: CallbackA) => number;
    b: (arg1: string, callback: CallbackB) => number;
}

declare const functions: Functions;

export function create<T extends keyof Functions>(key: T): Functions[T] {
    return (arg1: string, cb: CallbackA | CallbackA) => functions[key].call(undefined, arg1, cb)
}

declare let anyFunc: Functions[keyof Functions]

declare const cb: CallbackA | CallbackA;



anyFunc.call(undefined, "", cb)

🙁 Actual behavior

The error message says:

The 'this' context of type '((arg1: string, callback: CallbackA) => number) | ((arg1: string, callback: CallbackB) => number)' is not assignable to method's 'this' of type '(this: undefined, args_0: string, args_1: CallbackA) => number'.
  Type '(arg1: string, callback: CallbackB) => number' is not assignable to type '(this: undefined, args_0: string, args_1: CallbackA) => number'.
    Types of parameters 'callback' and 'args_1' are incompatible.
      Type 'CallbackA' is not assignable to type 'CallbackB'.
        Target signature provides too few arguments. Expected 2 or more, but got 1.(2684)

Or:

The 'this' context of type '((arg1: string, callback: CallbackA) => number) | ((arg1: string, callback: CallbackB) => number)' is not assignable to method's 'this' of type '(this: undefined, args_0: string, args_1: CallbackA) => number'.
  Type '(arg1: string, callback: CallbackB) => number' is not assignable to type '(this: undefined, args_0: string, args_1: CallbackA) => number'.(2684)

In neither example, this is not the problem.

In fact, if you add this: void to interface Functions and then cast to void, you get this:

The 'this' context of type '((this: void, arg1: string, callback: CallbackA) => number) | ((this: void, arg1: string, callback: CallbackB) => number)' is not assignable to method's 'this' of type '(this: void, args_0: string, args_1: CallbackA) => number'.
  Type '(this: void, arg1: string, callback: CallbackB) => number' is not assignable to type '(this: void, args_0: string, args_1: CallbackA) => number'.
    Types of parameters 'callback' and 'args_1' are incompatible.
      Type 'CallbackA' is not assignable to type 'CallbackB'.(2684)

Or even undefined:

The 'this' context of type '((this: undefined, arg1: string, callback: CallbackA) => number) | ((this: undefined, arg1: string, callback: CallbackB) => number)' is not assignable to method's 'this' of type '(this: undefined, args_0: string, args_1: CallbackA) => number'.
  Type '(this: undefined, arg1: string, callback: CallbackB) => number' is not assignable to type '(this: undefined, args_0: string, args_1: CallbackA) => number'.(2684)

None of these makes sense.

🙂 Expected behavior

The message doesn't claim that this is the problem; the problem is in other params.

If you go back to 4.4, you get a more reasonable error that simply complains about the callback param:

Argument of type 'CallbackA' is not assignable to parameter of type 'CallbackB'.(2345)

Additional information about the issue

Noticed this while attempting to enable strictBindCallApply on our repo.