fix: ensure if block paths retain correct template namespacing (#14685)

* fix: ensure if block paths retain correct template namespacing

* add tests

* address feedback

* address feedback

* simplify

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

Author: trueadm

.changeset/giant-moons-accept.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure if block paths retain correct template namespacing

packages/svelte/src/compiler/phases/3-transform/utils.js

@@ -347,7 +347,24 @@ export function infer_namespace(namespace, parent, nodes) {
 		}
 	}
 
-	return namespace;
+	/** @type {Namespace | null} */
+	let new_namespace = null;
+
+	// Check the elements within the fragment and look for consistent namespaces.
+	// If we have no namespaces or they are mixed, then fallback to existing namespace
+	for (const node of nodes) {
+		if (node.type !== 'RegularElement') continue;
+
+		if (node.metadata.mathml) {
+			new_namespace = new_namespace === null || new_namespace === 'mathml' ? 'mathml' : 'html';
+		} else if (node.metadata.svg) {
+			new_namespace = new_namespace === null || new_namespace === 'svg' ? 'svg' : 'html';
+		} else {
+			return 'html';
+		}
+	}
+
+	return new_namespace ?? namespace;
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/svg-namespace-if-block/Child.svelte

@@ -0,0 +1,8 @@
+
+{#if true}
+	<g>
+		<rect x="20" y="10" width="50" height="50" fill="yellow"/>
+	</g>
+{:else}
+	<div>lol</div>
+{/if}

packages/svelte/tests/runtime-runes/samples/svg-namespace-if-block/_config.js

@@ -0,0 +1,14 @@
+import { test, ok } from '../../test';
+
+export default test({
+	html: `<svg height="200px" viewBox="0 0 100 100" width="200px"><g><rect fill="yellow" height="50" width="50" x="20" y="10"></rect></g></svg>`,
+	test({ assert, target }) {
+		const g = target.querySelector('g');
+		const rect = target.querySelector('rect');
+		ok(g);
+		ok(rect);
+
+		assert.equal(g.namespaceURI, 'http://www.w3.org/2000/svg');
+		assert.equal(rect.namespaceURI, 'http://www.w3.org/2000/svg');
+	}
+});

packages/svelte/tests/runtime-runes/samples/svg-namespace-if-block/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Child from "./Child.svelte";
+</script>
+
+<svg viewBox="0 0 100 100" width="200px" height="200px">
+	<Child />
+</svg>