From 0e0f01ee1cfe11461f799c034030840a1b7cd763 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 27 Jan 2025 08:13:13 -0500
Subject: [PATCH] fix: disallow $state/$derived in const tags (#15115)

---
 .changeset/strong-shoes-whisper.md                 |  5 +++++
 .../phases/2-analyze/visitors/CallExpression.js    |  3 ++-
 .../const-tag-invalid-rune-usage/errors.json       | 14 ++++++++++++++
 .../const-tag-invalid-rune-usage/input.svelte      |  3 +++
 4 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/strong-shoes-whisper.md
 create mode 100644 packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json
 create mode 100644 packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/input.svelte

diff --git a/.changeset/strong-shoes-whisper.md b/.changeset/strong-shoes-whisper.md
new file mode 100644
index 0000000000..ea7f8d6c5e
--- /dev/null
+++ b/.changeset/strong-shoes-whisper.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: disallow $state/$derived in const tags
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js
index 96788d9f93..0a6b3f3ee5 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js
@@ -79,7 +79,8 @@ export function CallExpression(node, context) {
 		case '$derived':
 		case '$derived.by':
 			if (
-				parent.type !== 'VariableDeclarator' &&
+				(parent.type !== 'VariableDeclarator' ||
+					get_parent(context.path, -3).type === 'ConstTag') &&
 				!(parent.type === 'PropertyDefinition' && !parent.static && !parent.computed)
 			) {
 				e.state_invalid_placement(node, rune);
diff --git a/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json b/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json
new file mode 100644
index 0000000000..32594e4268
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_invalid_placement",
+		"message": "`$derived(...)` can only be used as a variable declaration initializer or a class field",
+		"start": {
+			"line": 2,
+			"column": 15
+		},
+		"end": {
+			"line": 2,
+			"column": 26
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/input.svelte b/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/input.svelte
new file mode 100644
index 0000000000..a056058cc5
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/input.svelte
@@ -0,0 +1,3 @@
+{#snippet test()}
+	{@const der = $derived(0)}
+{/snippet}
\ No newline at end of file