From 9adebfd71950d722ec878cf9d51c19b5e8de8818 Mon Sep 17 00:00:00 2001
From: Nic <162764842+Nic-Polumeyv@users.noreply.github.com>
Date: Fri, 18 Sep 2026 21:20:35 -0400
Subject: [PATCH] fix: error at compile time when a declaration in a snippet
 redeclares one of its parameters

---
 .changeset/snippet-parameter-redeclared.md         |  5 +++++
 packages/svelte/src/compiler/phases/scope.js       |  8 ++++++++
 .../declaration-tag-snippet-parameter/errors.json  | 14 ++++++++++++++
 .../declaration-tag-snippet-parameter/input.svelte |  6 ++++++
 .../errors.json                                    |  1 +
 .../input.svelte                                   |  8 ++++++++
 6 files changed, 42 insertions(+)
 create mode 100644 .changeset/snippet-parameter-redeclared.md
 create mode 100644 packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/errors.json
 create mode 100644 packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/input.svelte
 create mode 100644 packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/errors.json
 create mode 100644 packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/input.svelte

diff --git a/.changeset/snippet-parameter-redeclared.md b/.changeset/snippet-parameter-redeclared.md
new file mode 100644
index 0000000000..9bb14cf3af
--- /dev/null
+++ b/.changeset/snippet-parameter-redeclared.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error at compile time when a declaration in a snippet redeclares one of its parameters
diff --git a/packages/svelte/src/compiler/phases/scope.js b/packages/svelte/src/compiler/phases/scope.js
index 77c9119364..0d8e570772 100644
--- a/packages/svelte/src/compiler/phases/scope.js
+++ b/packages/svelte/src/compiler/phases/scope.js
@@ -1343,6 +1343,14 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 			}
 
 			context.next({ scope: child_scope });
+
+			// the parameters and what the body declares end up in the same function
+			const body_scope = /** @type {Scope} */ (scopes.get(node.body));
+			for (const [name, binding] of body_scope.declarations) {
+				if (child_scope.declarations.has(name)) {
+					e.declaration_duplicate(binding.node, name);
+				}
+			}
 		},
 
 		Fragment: (node, context) => {
diff --git a/packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/errors.json b/packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/errors.json
new file mode 100644
index 0000000000..a5df63547c
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "declaration_duplicate",
+		"message": "`item` has already been declared",
+		"start": {
+			"line": 2,
+			"column": 8
+		},
+		"end": {
+			"line": 2,
+			"column": 12
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/input.svelte b/packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/input.svelte
new file mode 100644
index 0000000000..800a3ae8d1
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/input.svelte
@@ -0,0 +1,6 @@
+{#snippet row(item)}
+	{const item = 1}
+	<p>{item}</p>
+{/snippet}
+
+{@render row(5)}
diff --git a/packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/errors.json b/packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/errors.json
new file mode 100644
index 0000000000..fe51488c70
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/errors.json
@@ -0,0 +1 @@
+[]
diff --git a/packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/input.svelte b/packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/input.svelte
new file mode 100644
index 0000000000..90162859a1
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/input.svelte
@@ -0,0 +1,8 @@
+{#snippet row(item)}
+	{#if item}
+		{const item = 1}
+		<p>{item}</p>
+	{/if}
+{/snippet}
+
+{@render row(5)}