fix: error at compile time when a declaration in a snippet redeclares one of its parameters

.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

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) => {

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
+		}
+	}
+]

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)}

packages/svelte/tests/validator/samples/snippet-parameter-shadowed-in-block/errors.json

@@ -0,0 +1 @@
+[]

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)}