[q] Allow thenReject to return a promise for a different type (#23029)

* Change the Promise.thenReject signature to allow matching of a
  different type.

Type-checking rejections is problematic as it's equivalent to `throw`:
you don't get a value of type T in your success callback. You get
an Error (or other thrown type) in an error callback.

types/q/index.d.ts

@@ -145,7 +145,7 @@ declare namespace Q {
 		/**
 		 * A sugar method, equivalent to promise.then(function () { throw reason; }).
 		 */
-		thenReject(reason?: any): Promise<T>;
+		thenReject<U = T>(reason?: any): Promise<U>;
 
 		/**
 		 * Attaches a handler that will observe the value of the promise when it becomes fulfilled, returning a promise for that same value, perhaps deferred but not replaced by the promise returned

types/q/q-tests.ts

@@ -234,3 +234,37 @@ Q.try(() => {
 	return true;
 })
 	.catch((error) => console.error("Couldn't sync to the cloud", error));
+
+// thenReject, returning a Promise of the same type as the Promise it is called on
+function thenRejectSameType(arg: any): Q.Promise<number> {
+	if (!arg) {
+		return returnsNumPromise('')
+			.thenReject(new Error('failed'));
+	}
+	return Q.resolve(2);
+}
+
+// thenReject, returning a Promise of a different type to the Promise it is called on.
+// The generic type argument is specified.
+function thenRejectSpecificOtherType(arg: any): Q.Promise<string> {
+	if (!arg) {
+		return returnsNumPromise('')
+			.thenReject<string>(new Error('failed'));
+	}
+	return Q.resolve('');
+}
+
+// thenReject, returning a Promise of a different type to the Promise it is called on.
+// The generic type argument is inferred.
+// This relies on 'Return types as inference targets', new in TS 2.4.
+// Commented out as we support TS 2.3.
+// This should be uncommented if the minimum version is changed.
+/*
+function thenRejectInferredOtherType(arg: any): Q.Promise<string> {
+	if (!arg) {
+		return returnsNumPromise('')
+			.thenReject(new Error('failed'));
+	}
+	return Q.resolve('');
+}
+*/