fix: error at compile time on duplicate snippet/declaration tag definitions (#18351)

.changeset/clever-poems-shave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error at compile time on duplicate snippet/declaration tag definitions

packages/svelte/src/compiler/phases/2-analyze/visitors/DeclarationTag.js

@@ -2,6 +2,7 @@
 /** @import { Context } from '../types' */
 import * as b from '#compiler/builders';
 import * as e from '../../../errors.js';
+import { extract_identifiers } from '../../../utils/ast.js';
 
 /**
  * @param {AST.DeclarationTag} node
@@ -12,6 +13,16 @@ export function DeclarationTag(node, context) {
 		e.declaration_tag_no_legacy_mode(node);
 	}
 
+	const is_top_level = context.path.length === 1 && context.path[0].type === 'Fragment';
+	if (is_top_level) {
+		const duplicate = node.declaration.declarations
+			.flatMap((declaration) => extract_identifiers(declaration.id))
+			.find((id) => context.state.analysis.instance.scope.declarations.has(id.name));
+		if (duplicate) {
+			e.declaration_duplicate(duplicate, duplicate.name);
+		}
+	}
+
 	context.visit(node.declaration, {
 		...context.state,
 		in_declaration_tag: true,

packages/svelte/src/compiler/phases/2-analyze/visitors/SnippetBlock.js

@@ -25,19 +25,24 @@ export function SnippetBlock(node, context) {
 
 	context.next({ ...context.state, parent_element: null });
 
-	const can_hoist =
+	const is_top_level = context.path.length === 1 && context.path[0].type === 'Fragment';
-		context.path.length === 1 &&
-		context.path[0].type === 'Fragment' &&
-		can_hoist_snippet(context.state.scope, context.state.scopes);
 
-	const name = node.expression.name;
+	if (is_top_level) {
+		const name = node.expression.name;
 
-	if (can_hoist) {
+		if (context.state.analysis.instance.scope.declarations.has(name)) {
-		const binding = /** @type {Binding} */ (context.state.scope.get(name));
+			e.declaration_duplicate(node.expression, name);
-		context.state.analysis.module.scope.declarations.set(name, binding);
+		}
-	}
+
+		node.metadata.can_hoist =
+			is_top_level && can_hoist_snippet(context.state.scope, context.state.scopes);
 
-	node.metadata.can_hoist = can_hoist;
+		if (node.metadata.can_hoist) {
+			const name = node.expression.name;
+			const binding = /** @type {Binding} */ (context.state.scope.get(name));
+			context.state.analysis.module.scope.declarations.set(name, binding);
+		}
+	}
 
 	const { path } = context;
 	const parent = path.at(-2);

packages/svelte/tests/validator/samples/declaration-tag-already-declared/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "declaration_duplicate",
+		"message": "`foo` has already been declared",
+		"start": {
+			"line": 5,
+			"column": 5
+		},
+		"end": {
+			"line": 5,
+			"column": 8
+		}
+	}
+]

packages/svelte/tests/validator/samples/declaration-tag-already-declared/input.svelte

@@ -0,0 +1,5 @@
+<script>
+	let foo = 'bar';
+</script>
+
+{let foo = 'baz'}

packages/svelte/tests/validator/samples/snippet-already-declared/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "declaration_duplicate",
+		"message": "`foo` has already been declared",
+		"start": {
+			"line": 5,
+			"column": 10
+		},
+		"end": {
+			"line": 5,
+			"column": 13
+		}
+	}
+]

packages/svelte/tests/validator/samples/snippet-already-declared/input.svelte

@@ -0,0 +1,5 @@
+<script>
+	let foo = 'bar';
+</script>
+
+{#snippet foo()}baz{/snippet}