Add base class in lib.scripthost.d.ts for Automation objects

[zspitz]

TypeScript Version: 2.5.0 / nightly (2.6.0-dev.20170902)

Code

// in lib.scripthost.d.ts
class AutomationObject<T> {
  private constructor();
  private typekey: AutomationObject<T>;
}

// in activex-excel.d.ts
declare namespace Excel {
    class Application extends AutomationObject<Application> {
        Workbooks: any;
    }
}

interface ActiveXObject {
    new(progid: 'Excel.Application'): Excel.Application;
}
    
// usage
let x: Excel.Application = new ActiveXObject('Excel.Application');

This currently wouldn't compile, because of the private constructor in the base class; but pending resolution of #18283, it would be possible.

This would prevent assigning structurally matching objects to the inheriting types:

// Compiler error
x = {
    Workbooks: [];
};

and also prevent using new to create a new instance of the inheriting type:

// Compiler error
x = new Excel.Application();

---

I've opened an issue to define similar behavior for VarDate and SafeArray<T> in lib.scripthost.d.ts. If AutomationObject<T> is defined, then it could be used in lib.scripthost.d.ts as a base class for these types as well:

declare class VarDate extends AutomationObject<VarDate> { }
declare class SafeArray<T> extends AutomationObject<SafeArray<T>> { }

[zspitz]

On further consideration, this suggestion would have value for a flat inheritance hierarchy, where all the declared classes inherit directly from AutomationObject<T>.

However, when one declared class inherits from another, inheriting from AutomationObject<T> would not prevent assigning an instance of the base class to a subclass typed variable or parameter:

declare class BaseAutomation extends AutomationObject<BaseAutomation> { }
declare class DerivedAutomation extends BaseAutomation { }
let x: BaseAutomation = ...
let y: DerivedAutomation = x; // both have the same private key, so it's a valid assignment

In the real world, this might not be an issue, because DerivedAutomation would probably have some additional members that would prevent this assignment.