Promise Safety

The following code doesn't error by default:

declare let both: Promise<{ type: string, value: string; }>;
declare let valueOnly: Promise<{ value: string }>;
declare let typeOnly: Promise<{ type: string }>;
declare let empty: Promise<{}>;

// Why no error?
both = valueOnly;
both = typeOnly;
both = empty;

This is because Promise<T> doesn't use the type T as a member and only in functions. The function type compatability rules allow the promise's to be compatible in this case.

Fix

You can add the following to increase Promise type safety (do it globally):

interface Promise<T>{
  _ensureTypeSafety: T;
}

And now the following errors:

declare let both: Promise<{ type: string, value: string; }>;
declare let valueOnly: Promise<{ value: string }>;
declare let typeOnly: Promise<{ type: string }>;
declare let empty: Promise<{}>;

// Error!
both = valueOnly; // property type is missing
both = typeOnly; // property value is missing
both = empty; // property missing