fix: update_branch with (anchor).data possible undefined on ios devices (#15851)

* fix: update_branch with (anchor).data possible undefined

* error sooner

* add tests

* changeset

---------

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

.changeset/silly-apples-remain.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle more hydration mismatches

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -12,6 +12,7 @@ import {
 	hydrate_next,
 	hydrate_node,
 	hydrating,
+	read_hydration_instruction,
 	remove_nodes,
 	set_hydrate_node,
 	set_hydrating
@@ -160,7 +161,7 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		let mismatch = false;
 
 		if (hydrating) {
-			var is_else = /** @type {Comment} */ (anchor).data === HYDRATION_START_ELSE;
+			var is_else = read_hydration_instruction(anchor) === HYDRATION_START_ELSE;
 
 			if (is_else !== (length === 0)) {
 				// hydration mismatch — remove the server-rendered DOM and start over

packages/svelte/src/internal/client/dom/blocks/if.js

@@ -4,6 +4,7 @@ import {
 	hydrate_next,
 	hydrate_node,
 	hydrating,
+	read_hydration_instruction,
 	remove_nodes,
 	set_hydrate_node,
 	set_hydrating
@@ -56,7 +57,8 @@ export function if_block(node, fn, [root_index, hydrate_index] = [0, 0]) {
 
 		if (hydrating && hydrate_index !== -1) {
 			if (root_index === 0) {
-				const data = /** @type {Comment} */ (anchor).data;
+				const data = read_hydration_instruction(anchor);
+
 				if (data === HYDRATION_START) {
 					hydrate_index = 0;
 				} else if (data === HYDRATION_START_ELSE) {

packages/svelte/src/internal/client/dom/hydration.js

@@ -103,3 +103,16 @@ export function remove_nodes() {
 		node = next;
 	}
 }
+
+/**
+ *
+ * @param {TemplateNode} node
+ */
+export function read_hydration_instruction(node) {
+	if (!node || node.nodeType !== 8) {
+		w.hydration_mismatch();
+		throw HYDRATION_ERROR;
+	}
+
+	return /** @type {Comment} */ (node).data;
+}

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15819
+export default test({
+	expect_hydration_error: true
+});

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html

@@ -0,0 +1 @@
+<p>start</p><p>cond</p><!---->

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html

@@ -0,0 +1 @@
+<!--[--> <p>start</p><!--[--><p>cond</p><!--]--><!--]-->

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	const cond = true;
+</script>
+
+<p>start</p>{#if cond}<p>cond</p>{/if}

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15819
+export default test({
+	expect_hydration_error: true
+});

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html

@@ -0,0 +1 @@
+<p>start</p> pre123 mid<!---->

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html

@@ -0,0 +1 @@
+<!--[--><p>start</p>pre<a>123</a> <!--[-->mid<!--]--><!--]-->

packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let cond = true;
+</script>
+
+<p>start</p>
+pre123
+{#if cond}
+mid
+{/if}