From bc8d8d3c93b6ecb97de2b8fb808e5187f051ae9d Mon Sep 17 00:00:00 2001
From: divdavem <david-emmanuel.divernois@amadeus.com>
Date: Fri, 23 Sep 2022 12:52:23 +0200
Subject: [PATCH] fix: correctly infer the type of the function passed to
 derived

Before this commit, the type of the function passed to derived was not
correctly inferred by typescript in the async case. By putting the async
case first in the list of overloads of the derived function, it looks like
it fixes the issue, and the type is still correctly inferred in the sync
case too.
---
 src/index.spec.ts | 20 ++++++++++++++++++++
 src/index.ts      |  8 ++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/src/index.spec.ts b/src/index.spec.ts
index de468a6..096785f 100644
--- a/src/index.spec.ts
+++ b/src/index.spec.ts
@@ -971,6 +971,26 @@ describe('stores', () => {
       });
     });
 
+    it('should infer types automatically in the async case', () => {
+      const a = writable(1);
+      const b = writable(2);
+      const sum = derived(
+        [a, b],
+        ([a, b], set) => {
+          set(a + b);
+        },
+        0
+      );
+      expect(get(sum)).toBe(3);
+    });
+
+    it('should infer types automatically in the sync case', () => {
+      const a = writable(1);
+      const b = writable(2);
+      const sum = derived([a, b], ([a, b]) => a + b);
+      expect(get(sum)).toBe(3);
+    });
+
     it('should call clean-up function returned in deriveFn with derived', () => {
       const a = writable(1);
       const cleanUpFn = jasmine.createSpy('cleanupFn').and.callThrough();
diff --git a/src/index.ts b/src/index.ts
index a6a5a7c..bbcd018 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -752,13 +752,13 @@ export abstract class DerivedStore<
  */
 export function derived<T, S extends SubscribableStores>(
   stores: S,
-  options: SyncDeriveFn<T, S> | SyncDeriveOptions<T, S>,
-  initialValue?: T
+  options: AsyncDeriveFn<T, S> | AsyncDeriveOptions<T, S>,
+  initialValue: T
 ): Readable<T>;
 export function derived<T, S extends SubscribableStores>(
   stores: S,
-  options: AsyncDeriveFn<T, S> | AsyncDeriveOptions<T, S>,
-  initialValue: T
+  options: SyncDeriveFn<T, S> | SyncDeriveOptions<T, S>,
+  initialValue?: T
 ): Readable<T>;
 export function derived<T, S extends SubscribableStores>(
   stores: S,