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

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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.

Reproduce the strictBindCallApply diagnostic from the linked TypeScript Playground, comparing versions 4.4 and 4.5. Trace the diagnostic for the calls involving Functions[keyof Functions] and verify that the final message identifies the incompatible callback parameter rather than blaming the this context.