From 4f4b76f51fce2b3cda8fe34bf5eec2627c9709ff Mon Sep 17 00:00:00 2001
From: Nic <162764842+Nic-Polumeyv@users.noreply.github.com>
Date: Fri, 17 Jul 2026 22:02:52 -0400
Subject: [PATCH] fix: allow transitions to return a function taking a
 `direction` argument

Custom transitions may return `(options: { direction: 'in' | 'out' }) => TransitionConfig`,
which Svelte supports, but the `__sveltets_2_SvelteTransitionReturnType` shim only allowed
`TransitionConfig | (() => TransitionConfig)`. A function with a required parameter is not
assignable to `() => TransitionConfig`, so valid custom transitions were reported as errors.

Widen the shim's return type to match Svelte's, in both the v4 and v3 shims. The added
transition-options fixture fails without the change.

Fixes #2686
---
 .changeset/transition-direction-return.md                | 6 ++++++
 .../diagnostics/fixtures/transition-options/input.svelte | 9 +++++++++
 packages/svelte2tsx/svelte-shims-v4.d.ts                 | 4 ++--
 packages/svelte2tsx/svelte-shims.d.ts                    | 4 ++--
 4 files changed, 19 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/transition-direction-return.md

diff --git a/.changeset/transition-direction-return.md b/.changeset/transition-direction-return.md
new file mode 100644
index 000000000..170517a07
--- /dev/null
+++ b/.changeset/transition-direction-return.md
@@ -0,0 +1,6 @@
+---
+'svelte-check': patch
+'svelte2tsx': patch
+---
+
+fix: allow transitions to return a function taking a `direction` argument
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/transition-options/input.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/transition-options/input.svelte
index b54423ea7..8c51e33ab 100644
--- a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/transition-options/input.svelte
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/transition-options/input.svelte
@@ -6,6 +6,15 @@
     ) {
       return {};
     }
+
+    function deferredTransition(
+      _node: HTMLElement,
+      _params: { delay: number },
+      _context: { direction: 'in' | 'out' | 'both' }
+    ) {
+      return (_options: { direction: 'in' | 'out' }) => ({ delay: 100 });
+    }
 </script>
 
 <div in:myTransition={{ delay: 100 }} />
+<div in:deferredTransition={{ delay: 100 }} />
diff --git a/packages/svelte2tsx/svelte-shims-v4.d.ts b/packages/svelte2tsx/svelte-shims-v4.d.ts
index 29d84a2d9..0be208945 100644
--- a/packages/svelte2tsx/svelte-shims-v4.d.ts
+++ b/packages/svelte2tsx/svelte-shims-v4.d.ts
@@ -22,7 +22,7 @@ type SvelteTransitionConfig = {
 }
 
 /** @internal PRIVATE API, DO NOT USE */
-type SvelteTransitionReturnType = SvelteTransitionConfig | (() => SvelteTransitionConfig)
+type SvelteTransitionReturnType = SvelteTransitionConfig | ((options: { direction: 'in' | 'out' }) => SvelteTransitionConfig)
 
 /** @internal PRIVATE API, DO NOT USE */
 type SvelteAnimationReturnType = {
@@ -173,7 +173,7 @@ type __sveltets_2_SvelteTransitionConfig = {
     tick?: (t: number, u: number) => void
 }
 /** @internal PRIVATE API, DO NOT USE */
-type __sveltets_2_SvelteTransitionReturnType = __sveltets_2_SvelteTransitionConfig | (() => __sveltets_2_SvelteTransitionConfig)
+type __sveltets_2_SvelteTransitionReturnType = __sveltets_2_SvelteTransitionConfig | ((options: { direction: 'in' | 'out' }) => __sveltets_2_SvelteTransitionConfig)
 declare function __sveltets_2_ensureTransition(transitionCall: __sveltets_2_SvelteTransitionReturnType): {};
 
 // Includes undefined and null for all types as all usages also allow these
diff --git a/packages/svelte2tsx/svelte-shims.d.ts b/packages/svelte2tsx/svelte-shims.d.ts
index b7be2cbcc..f8bbc01ca 100644
--- a/packages/svelte2tsx/svelte-shims.d.ts
+++ b/packages/svelte2tsx/svelte-shims.d.ts
@@ -103,7 +103,7 @@ type SvelteTransitionConfig = {
 }
 
 /** @internal PRIVATE API, DO NOT USE */
-type SvelteTransitionReturnType = SvelteTransitionConfig | (() => SvelteTransitionConfig)
+type SvelteTransitionReturnType = SvelteTransitionConfig | ((options: { direction: 'in' | 'out' }) => SvelteTransitionConfig)
 
 /** @internal PRIVATE API, DO NOT USE */
 type SvelteAnimationReturnType = {
@@ -253,7 +253,7 @@ type __sveltets_2_SvelteTransitionConfig = {
     tick?: (t: number, u: number) => void
 }
 /** @internal PRIVATE API, DO NOT USE */
-type __sveltets_2_SvelteTransitionReturnType = __sveltets_2_SvelteTransitionConfig | (() => __sveltets_2_SvelteTransitionConfig)
+type __sveltets_2_SvelteTransitionReturnType = __sveltets_2_SvelteTransitionConfig | ((options: { direction: 'in' | 'out' }) => __sveltets_2_SvelteTransitionConfig)
 declare function __sveltets_2_ensureTransition(transitionCall: __sveltets_2_SvelteTransitionReturnType): {};
 
 // Includes undefined and null for all types as all usages also allow these