Only the last signature of an overloaded function is allowed in Parameters<T>

[calvinlauyh]

TypeScript Version: 3.2.1

Search Terms: overloaded function. overloaded signature, overloaded arguments

Code

function testFn(a: string): boolean;
function testFn(a: number): boolean;
function testFn(a: boolean): boolean;
function testFn(_a: string | number | boolean): boolean {
  return true;
}

interface Stub {
  callsFake(...args: Parameters<typeof testFn>): void
}

const stub1: Stub = {
  callsFake(_a: string): boolean {
    return true;
  }
}
const stub2: Stub = {
  callsFake(_a: number): boolean {
    return true;
  }
}
const stub3: Stub = {
  callsFake(_a: boolean): boolean {
    return true;
  }
}

Expected behavior:
Expected stub1 and stub2 to be compiled without any error.

Actual behavior:

Both stub1 and stub2 shows

Type '(_a: string) => boolean' is not assignable to type '(a: boolean) => any'.
  Types of parameters '_a' and 'a' are incompatible.
    Type 'boolean' is not assignable to type 'string'. [2322]

Playground Link:
http://www.typescriptlang.org/play/#src=function%20testFn(a%3A%20string)%3A%20boolean%3B%0D%0Afunction%20testFn(a%3A%20number)%3A%20boolean%3B%0D%0Afunction%20testFn(a%3A%20boolean)%3A%20boolean%3B%0D%0Afunction%20testFn(_a%3A%20string%20%7C%20number%20%7C%20boolean)%3A%20boolean%20%7B%0D%0A%20%20return%20true%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20Stub%20%7B%0D%0A%20%20callsFake(...args%3A%20Parameters%3Ctypeof%20testFn%3E)%3A%20void%0D%0A%7D%0D%0A%0D%0Aconst%20stub1%3A%20Stub%20%3D%20%7B%0D%0A%20%20callsFake(_a%3A%20string)%3A%20boolean%20%7B%0D%0A%20%20%20%20return%20true%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0Aconst%20stub2%3A%20Stub%20%3D%20%7B%0D%0A%20%20callsFake(_a%3A%20number)%3A%20boolean%20%7B%0D%0A%20%20%20%20return%20true%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0Aconst%20stub3%3A%20Stub%20%3D%20%7B%0D%0A%20%20callsFake(_a%3A%20boolean)%3A%20boolean%20%7B%0D%0A%20%20%20%20return%20true%3B%0D%0A%20%20%7D%0D%0A%7D

Related Issues:
#13570
The closest I can find is this but it has generic types at the first function's signature, I am not sure fundamentally do they refer to the same issue.

[MartinJohns]

From the documentation for "Conditional Types":

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types.

https://www.typescriptlang.org/docs/handbook/advanced-types.html
(section "Type inference in conditional types", direct linking unfortunately not possible)

[calvinlauyh]

Thanks, @MartinJohns
Closing this issue, and also providing the direct link for future reference:
https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-inference-in-conditional-types

[MartinJohns]

Oh, direct linking is possible! I did not bother check the markup, just assumed there would be a visible anchor.