MethodDecorator gets TS2322/TS2315

TypeScript Version: 2.4.2 Code

export function route(method: string, path: string): MethodDecorator {
    return (target: any, name: string, descriptor: TypedPropertyDescriptor<Function>) => {
        routeManager.regeisterRoute({
            constructor: target.constructor,
            function: descriptor.value,
            method,
            path
        });
    }
}

export function route(method: string, path: string): MethodDecorator<Function> {
    return (target: any, name: string, descriptor: TypedPropertyDescriptor<Function>) => {
        routeManager.regeisterRoute({
            constructor: target.constructor,
            function: descriptor.value,
            method,
            path
        });
    }
}

Expected behavior: No error Actual behavior:

error TS2322: Type '(target: any, name: string, descriptor: TypedPropertyDescriptor<Function>) => void' is not assignable to type 'MethodDecorate
or'.
  Types of parameters 'descriptor' and 'descriptor' are incompatible.
    Type 'TypedPropertyDescriptor<T>' is not assignable to type 'TypedPropertyDescriptor<Function>'.
      Type 'T' is not assignable to type 'Function'.

error TS2315: Type 'MethodDecorator' is not generic.

[ikatyang]

For the first one, it is caused by stricter generic checks (#16368).

It seems you have to define a CustomMethodDecorator first, since there is no generic on MethodDecorator, and its declaration requires a non-constraint generic.

Or just enable the --noStrictGenericChecks option.

---

For the second one, there is indeed no generic on MethodDecorator, the error message seems a little weird.

(lib.d.ts)

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

[parzhitsky]

In case somebody needs an exact code, his is how I solved this issue in my project:

interface MethodDecoratorCustom<Instance extends object = object> {
    (
        target: Instance | Constructor<Instance>,
        key: PropertyKey,
        descriptor: TypedPropertyDescriptor<Method<Instance | Constructor<Instance>>>,
    ): void | TypedPropertyDescriptor<Method<Instance | Constructor<Instance>>>;
}

(See definitions of Method and Constructor, and a usage example.)