From eea58df0a755458d29b604058ab50e77466a8523 Mon Sep 17 00:00:00 2001
From: Graham Rogers <graham@ophelos.com>
Date: Thu, 8 May 2025 18:00:39 +0100
Subject: [PATCH 1/3] Simplify the type of `MockActivityEnvironment.run`

This improves its type inference, see #1710
---
 packages/testing/src/mocking-activity-environment.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/testing/src/mocking-activity-environment.ts b/packages/testing/src/mocking-activity-environment.ts
index 8b1cec47b..ccb78f81f 100644
--- a/packages/testing/src/mocking-activity-environment.ts
+++ b/packages/testing/src/mocking-activity-environment.ts
@@ -54,7 +54,7 @@ export class MockActivityEnvironment extends events.EventEmitter {
   /**
    * Run a function in Activity Context
    */
-  public async run<P extends any[], R, F extends ActivityFunction<P, R>>(fn: F, ...args: P): Promise<R> {
+  public async run<F extends ActivityFunction>(fn: F, ...args: Parameters<F>): ReturnType<F> {
     return this.activity.runNoEncoding(fn as ActivityFunction<any, any>, { args, headers: {} }) as Promise<R>;
   }
 }

From 1701e50d8dbc18004ede6244c50004517a066332 Mon Sep 17 00:00:00 2001
From: Graham Rogers <graham@ophelos.com>
Date: Fri, 9 May 2025 11:00:29 +0100
Subject: [PATCH 2/3] Fix build

---
 packages/testing/src/mocking-activity-environment.ts | 4 ++--
 packages/worker/src/activity.ts                      | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/packages/testing/src/mocking-activity-environment.ts b/packages/testing/src/mocking-activity-environment.ts
index ccb78f81f..4d88adc22 100644
--- a/packages/testing/src/mocking-activity-environment.ts
+++ b/packages/testing/src/mocking-activity-environment.ts
@@ -54,8 +54,8 @@ export class MockActivityEnvironment extends events.EventEmitter {
   /**
    * Run a function in Activity Context
    */
-  public async run<F extends ActivityFunction>(fn: F, ...args: Parameters<F>): ReturnType<F> {
-    return this.activity.runNoEncoding(fn as ActivityFunction<any, any>, { args, headers: {} }) as Promise<R>;
+  public run<F extends ActivityFunction>(fn: F, ...args: Parameters<F>): ReturnType<F> {
+    return this.activity.runNoEncoding(fn, { args, headers: {} }) as ReturnType<F>;
   }
 }
 
diff --git a/packages/worker/src/activity.ts b/packages/worker/src/activity.ts
index 8774dd3aa..3d7720a05 100644
--- a/packages/worker/src/activity.ts
+++ b/packages/worker/src/activity.ts
@@ -164,9 +164,9 @@ export class Activity {
     });
   }
 
-  public runNoEncoding(fn: ActivityFunction<any[], any>, input: ActivityExecuteInput): Promise<unknown> {
+  public runNoEncoding<F extends ActivityFunction>(fn: F, input: ActivityExecuteInput): ReturnType<F> {
     if (this.fn !== undefined) throw new IllegalStateError('Activity function is defined');
-    return asyncLocalStorage.run(this.context, () => this.execute(fn, input));
+    return asyncLocalStorage.run(this.context, () => this.execute(fn, input)) as ReturnType<F>;
   }
 }
 

From ba5976c47e0e4aca12445a4922043bcba6fe640e Mon Sep 17 00:00:00 2001
From: Graham Rogers <graham@ophelos.com>
Date: Fri, 9 May 2025 11:07:15 +0100
Subject: [PATCH 3/3] Add return type test

---
 packages/test/src/test-mockactivityenv.ts | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/packages/test/src/test-mockactivityenv.ts b/packages/test/src/test-mockactivityenv.ts
index 6811cf06b..cfdc2eb35 100644
--- a/packages/test/src/test-mockactivityenv.ts
+++ b/packages/test/src/test-mockactivityenv.ts
@@ -47,3 +47,13 @@ test('MockActivityEnvironment injects provided info', async (t) => {
   }, 1);
   t.is(res, 4);
 });
+
+test('MockActivityEnvironment return type is correctly inferred', async (t) => {
+  async function foo(): Promise<string> {
+    return 'foo';
+  }
+  const result = await new MockActivityEnvironment().run(foo);
+  // There should be no compile time error on this line
+  result.startsWith('foo');
+  t.pass();
+});