fix: ensure if block paths retain correct template namespacing

.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/client/visitors/IfBlock.js

@@ -1,8 +1,42 @@
 /** @import { BlockStatement, Expression } from 'estree' */
-/** @import { AST } from '#compiler' */
+/** @import { AST, Namespace } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
 import * as b from '../../../../utils/builders.js';
 
+/**
+ * @param {AST.Fragment} fragment
+ * @return {Namespace}
+ */
+function get_namespace(fragment) {
+	const elements = fragment.nodes.filter((n) => n.type === 'RegularElement');
+	/** @type {Namespace | null} */
+	let 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 `html`
+	for (const element of elements) {
+		const metadata = element.metadata;
+
+		if (metadata.mathml) {
+			if (namespace === null || namespace === 'mathml') {
+				namespace = 'mathml';
+			} else {
+				namespace = 'html';
+			}
+		} else if (metadata.svg) {
+			if (namespace === null || namespace === 'svg') {
+				namespace = 'svg';
+			} else {
+				namespace = 'html';
+			}
+		} else {
+			namespace = 'html';
+		}
+	}
+
+	return namespace ?? 'html';
+}
+
 /**
  * @param {AST.IfBlock} node
  * @param {ComponentContext} context
@@ -11,7 +45,13 @@ export function IfBlock(node, context) {
 	context.state.template.push('<!>');
 	const statements = [];
 
-	const consequent = /** @type {BlockStatement} */ (context.visit(node.consequent));
+	const consequent_namespace = get_namespace(node.consequent);
+	const consequent = /** @type {BlockStatement} */ (
+		context.visit(node.consequent, {
+			...context.state,
+			metadata: { ...context.state.metadata, namespace: consequent_namespace }
+		})
+	);
 	const consequent_id = context.state.scope.generate('consequent');
 
 	statements.push(b.var(b.id(consequent_id), b.arrow([b.id('$$anchor')], consequent)));
@@ -19,7 +59,13 @@ export function IfBlock(node, context) {
 	let alternate_id;
 
 	if (node.alternate) {
-		const alternate = /** @type {BlockStatement} */ (context.visit(node.alternate));
+		const alternate_namespace = get_namespace(node.consequent);
+		const alternate = /** @type {BlockStatement} */ (
+			context.visit(node.alternate, {
+				...context.state,
+				metadata: { ...context.state.metadata, namespace: alternate_namespace }
+			})
+		);
 		alternate_id = context.state.scope.generate('alternate');
 		statements.push(b.var(b.id(alternate_id), b.arrow([b.id('$$anchor')], alternate)));
 	}