fix(workflow): make ExternalWorkflowHandle.signal signature match that of BaseWorkflowHandle.signal (#1237)

Co-authored-by: Gabriel Santos-Blanchet <gsantosblanchet@progymedia.com>

Author: mjameswh

packages/test/src/test-interface-type-safety.ts

@@ -1,5 +1,6 @@
 import test from 'ava';
-import { defineSignal, defineQuery } from '@temporalio/workflow';
+import { defineSignal, defineQuery, ExternalWorkflowHandle, ChildWorkflowHandle, Workflow } from '@temporalio/workflow';
+import { WorkflowHandle } from '@temporalio/client';
 
 test('SignalDefinition Name type safety', (t) => {
   // @ts-expect-error Assert expect a type error when generic and concrete names do not match
@@ -53,3 +54,15 @@ test('QueryDefinition Args and Ret type safety', (t) => {
   const _argAssertion: ArgTypeAssertion = 'not-intermixable';
   t.pass();
 });
+
+test('Can call signal on any WorkflowHandle', async (t) => {
+  // This function definition is an assertion by itself. TSC will throw a compile time error if
+  // the signature of the signal function is not compatible across all WorkflowHandle variants.
+  async function _assertion<T extends Workflow>(
+    handle: WorkflowHandle<T> | ChildWorkflowHandle<T> | ExternalWorkflowHandle
+  ) {
+    await handle.signal(defineSignal('signal'));
+  }
+
+  t.pass();
+});

packages/workflow/src/workflow-handle.ts

@@ -17,7 +17,10 @@ export interface ExternalWorkflowHandle {
    * await handle.signal(incrementSignal, 3);
    * ```
    */
-  signal<Args extends any[] = []>(def: SignalDefinition<Args> | string, ...args: Args): Promise<void>;
+  signal<Args extends any[] = [], Name extends string = string>(
+    def: SignalDefinition<Args, Name> | string,
+    ...args: Args
+  ): Promise<void>;
 
   /**
    * Cancel the external Workflow execution.