Establishing the Function Pointer Parameter Type for Method Overload in TypeScript

I encountered a compile error that I can't quite figure out. (Just to clarify, this issue only occurs in TS 2.4.1, while TS 2.3.4 works perfectly fine.) Below is a sample code snippet showcasing the compile error. Although creating an overload is not recommended in this scenario and using a union type for the function signature would be more appropriate, I chose to stick with a simpler example to highlight the problem:

class Animal {
    a: string;
}

class Dog extends Animal {
    b: string;
}

function test(func: (p: Dog) => any): any;
function test(func: (p: Animal) => any): any;
function test(func: (p: Animal) => any): any {

    return undefined;
}

The compiler throws a TS2394 error:

Overload signature is not compatible with function implementation

This error specifically occurs at:

function test(func: (p: Dog) => any): any;

The error can be resolved by adjusting the function definition as follows:

function test(func: (p: any) => any): any {

Answer â„–1

With help from Joe Calzaretta (@jcalz), I finally cracked the code on this (https://github.com/Microsoft/TypeScript/issues/17162).

The introduction of Strict contravariance for callback parameters in version 2.4 is a game-changer. Here's a snippet of wisdom from Joe:

If we label (p: Dog) => any as a Dog-consumer, and (p: Animal) => any as an Animal-consumer, it's important to note that every Animal-consumer can also consume a Dog (since something designed to handle animals should be able to handle dogs). On the other hand, not every Dog-consumer can handle an Animal (as something tailored for dogs may not work for cats). This hierarchy implies that the Dog-consumer is a supertype of the Animal-consumer. Consequently, when one overload expects a Dog-consumer and the other anticipates an Animal-consumer, the implementation should cater to their common ancestor: the Dog-consumer.

function test(func: (p: Dog) => any): any;
function test(func: (p: Animal) => any): any;
function test(func: (p: Dog | Animal) => any): any {

The initial approach fails because you can't pass an Animal to the Dog Consumer—it specifically requires a dog. The most precise representation would look like this:

function test(func: (p: Dog) => any): any;
function test(func: (p: Animal) => any): any;
function test(func: ((p: Animal) => any) | ((p:Dog) => any)): any {

This setup allows func to accept either a function targeting animals or one focused on dogs.

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