As mentioned in #2249 (comment), consider this:

// decorator factory
function F(options?: {optional: boolean}) {
     // returns a property decorator
    return (target , propertyKey): void => {
        // decorator implementation
    }
}

class C {
    @F()    // OK, used as a factory
    prop1; 

    @F   // No error
    prop2; 
}

The problem here is that the compiler compares the signature of the decorator factory F against declare type PropertyDecorator = (target: Object, propertyKey: string | symbol) => void; which ends up to be assignable. I think we need a stronger check here to make sure we catch these issue.