Why can't Typescript infer the argument types of this function?

Question

Say we have this type declaration:

declare type MethodDecorator = <T>(target: Object, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor<T>) => TypedPropertyDescriptor<T> | void;

If we define a method which states this to be this type, why can't typescript deduce the types of the arguments?

const fetchJson: MethodDecorator = (target, propertyKey, descriptor) => {
    ...
}

Typescripts infer the arguments as having the any type. Why is this?

Answer 1

Contextual typing of function expressions does not occur when the contextual type's signature is generic. This is because it would cause the type parameter type to "leak out", which is a big no-no -- you should never be able to see an unspecified type parameter T outside the declaration in which it was declared.

You could fix this by moving the type parameter to the type instead of being on the signature :

type SomeMethodDecorator<T> = (target: Object, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor<T>) => TypedPropertyDescriptor<T> | void;

let x: SomeMethodDecorator<string> = (a, b, c) => {
    // a: Object, etc.
}