From a229d673078c7c53a00203abb3a4cac2cdd9eaca Mon Sep 17 00:00:00 2001
From: Brenley Dueck <brenleydueck@gmail.com>
Date: Sun, 14 Jun 2026 08:09:14 -0500
Subject: [PATCH 1/2] fix: improve createProjection seed inference

---
 .changeset/fix-create-projection-seed-inference.md | 6 ++++++
 packages/solid-signals/src/store/projection.ts     | 3 ++-
 packages/solid/src/client/hydration.ts             | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/fix-create-projection-seed-inference.md

diff --git a/.changeset/fix-create-projection-seed-inference.md b/.changeset/fix-create-projection-seed-inference.md
new file mode 100644
index 000000000..2b97fdca4
--- /dev/null
+++ b/.changeset/fix-create-projection-seed-inference.md
@@ -0,0 +1,6 @@
+---
+"@solidjs/signals": patch
+"solid-js": patch
+---
+
+Fix `createProjection` seed typing so readonly store seeds do not override inference from the projection function return type.
diff --git a/packages/solid-signals/src/store/projection.ts b/packages/solid-signals/src/store/projection.ts
index 3efd2c3a2..3a6d67edb 100644
--- a/packages/solid-signals/src/store/projection.ts
+++ b/packages/solid-signals/src/store/projection.ts
@@ -17,6 +17,7 @@ import {
   STORE_WRAP,
   storeSetter,
   storeTraps,
+  type NoFn,
   type ProjectionOptions,
   type Store
 } from "./store.js";
@@ -107,7 +108,7 @@ export function createProjectionInternal<T extends object = {}>(
  */
 export function createProjection<T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  seed: Partial<T>,
+  seed: NoFn<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ): Refreshable<Store<T>> {
   return createProjectionInternal(fn, seed, options).store;
diff --git a/packages/solid/src/client/hydration.ts b/packages/solid/src/client/hydration.ts
index 9acb5ac27..1c33ca4cb 100644
--- a/packages/solid/src/client/hydration.ts
+++ b/packages/solid/src/client/hydration.ts
@@ -1017,7 +1017,7 @@ export const createOptimistic: {
  */
 export const createProjection: <T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  initialValue: T,
+  initialValue: NoFn<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ) => Refreshable<Store<T>> = ((...args: any[]) =>
   (_createProjection || coreProjection)(...args)) as any;

From 0a93474861705c4bed52e34c7261e18b1322263f Mon Sep 17 00:00:00 2001
From: Brenley Dueck <brenleydueck@gmail.com>
Date: Thu, 2 Jul 2026 18:22:31 -0500
Subject: [PATCH 2/2] fix: keep partial seeds while allowing readonly store
 seeds

Match createStore's projection seed type (Partial<T> | Store<NoFn<T>>)
instead of NoFn<T> | Store<NoFn<T>>, which rejected the documented
partial-seed pattern. Align the server createProjection signature and
add type tests for store-as-seed inference.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 packages/solid-signals/src/store/projection.ts    |  2 +-
 .../solid-signals/tests/store/store.type-tests.ts | 15 +++++++++++++++
 packages/solid/src/client/hydration.ts            |  2 +-
 packages/solid/src/server/signals.ts              |  2 +-
 4 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/packages/solid-signals/src/store/projection.ts b/packages/solid-signals/src/store/projection.ts
index 3a6d67edb..44f739ee9 100644
--- a/packages/solid-signals/src/store/projection.ts
+++ b/packages/solid-signals/src/store/projection.ts
@@ -108,7 +108,7 @@ export function createProjectionInternal<T extends object = {}>(
  */
 export function createProjection<T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  seed: NoFn<T> | Store<NoFn<T>>,
+  seed: Partial<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ): Refreshable<Store<T>> {
   return createProjectionInternal(fn, seed, options).store;
diff --git a/packages/solid-signals/tests/store/store.type-tests.ts b/packages/solid-signals/tests/store/store.type-tests.ts
index 4a6cb2ecc..c1438ac00 100644
--- a/packages/solid-signals/tests/store/store.type-tests.ts
+++ b/packages/solid-signals/tests/store/store.type-tests.ts
@@ -98,6 +98,21 @@ import {
   store[0].name satisfies string;
 }
 
+// ── createProjection — store as seed ──────────────────────────────────
+
+{
+  const [todos] = createStore([] as { id: number; done: boolean }[]);
+  const proj = createProjection(() => todos.filter(t => !t.done), todos);
+  proj[0].id satisfies number;
+  proj[0].done satisfies boolean;
+}
+
+{
+  const [state] = createStore({ count: 0 });
+  const proj = createProjection(() => ({ count: 1 }), state);
+  proj.count satisfies number;
+}
+
 // ── createOptimisticStore (projection) — partial seed ─────────────────
 
 {
diff --git a/packages/solid/src/client/hydration.ts b/packages/solid/src/client/hydration.ts
index 1c33ca4cb..2d2455894 100644
--- a/packages/solid/src/client/hydration.ts
+++ b/packages/solid/src/client/hydration.ts
@@ -1017,7 +1017,7 @@ export const createOptimistic: {
  */
 export const createProjection: <T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  initialValue: NoFn<T> | Store<NoFn<T>>,
+  initialValue: Partial<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ) => Refreshable<Store<T>> = ((...args: any[]) =>
   (_createProjection || coreProjection)(...args)) as any;
diff --git a/packages/solid/src/server/signals.ts b/packages/solid/src/server/signals.ts
index ae203f7e6..122893f09 100644
--- a/packages/solid/src/server/signals.ts
+++ b/packages/solid/src/server/signals.ts
@@ -1056,7 +1056,7 @@ function createPendingProxy<T extends object>(
 
 export function createProjection<T extends object>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  initialValue: Partial<T>,
+  initialValue: Partial<T> | Store<T>,
   options?: ServerSsrOptions
 ): Store<T> {
   const ctx = sharedConfig.context;