From 8ef693e94eddca38d92c62357efed877afd41ddf Mon Sep 17 00:00:00 2001
From: Nic Polumeyv <nicolas.polum@outlook.com>
Date: Mon, 28 Sep 2026 16:55:39 -0400
Subject: [PATCH] fix: don't hang `migrate` on a declaration that shares a line
 with its script tag (#18816)

`migrate()` never returned when a declaration shared a line with its
`<script>` tag:

```svelte
<script>export let answer</script>
```

The scan that removes a migrated `export let` line looked for a newline
in both directions with no bound. It now stops at the script's edges.
---
 .changeset/migrate-tag-line.md                              | 5 +++++
 packages/svelte/src/compiler/migrate/index.js               | 6 ++++--
 .../migrate/samples/props-on-script-tag-line/input.svelte   | 2 ++
 .../migrate/samples/props-on-script-tag-line/output.svelte  | 3 +++
 4 files changed, 14 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/migrate-tag-line.md
 create mode 100644 packages/svelte/tests/migrate/samples/props-on-script-tag-line/input.svelte
 create mode 100644 packages/svelte/tests/migrate/samples/props-on-script-tag-line/output.svelte

diff --git a/.changeset/migrate-tag-line.md b/.changeset/migrate-tag-line.md
new file mode 100644
index 0000000000..414e4a249c
--- /dev/null
+++ b/.changeset/migrate-tag-line.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't hang `migrate` on a declaration that shares a line with its script tag
diff --git a/packages/svelte/src/compiler/migrate/index.js b/packages/svelte/src/compiler/migrate/index.js
index 3baef3ecae..00f997e054 100644
--- a/packages/svelte/src/compiler/migrate/index.js
+++ b/packages/svelte/src/compiler/migrate/index.js
@@ -910,8 +910,10 @@ const instance_script = {
 				start = /** @type {number} */ (parent.start);
 				end = /** @type {number} */ (parent.end);
 			}
-			while (state.str.original[start] !== '\n') start--;
-			while (state.str.original[end] !== '\n') end++;
+			// remove the declaration's whole line, or what there is of it inside the script
+			const script = /** @type {{ start: number; end: number }} */ (state.analysis.instance.ast);
+			while (start > script.start && state.str.original[start] !== '\n') start--;
+			while (end < script.end && state.str.original[end] !== '\n') end++;
 			state.str.update(start, end, '');
 		}
 	},
diff --git a/packages/svelte/tests/migrate/samples/props-on-script-tag-line/input.svelte b/packages/svelte/tests/migrate/samples/props-on-script-tag-line/input.svelte
new file mode 100644
index 0000000000..dd623897bd
--- /dev/null
+++ b/packages/svelte/tests/migrate/samples/props-on-script-tag-line/input.svelte
@@ -0,0 +1,2 @@
+<script>export let answer</script>
+<p>{answer}</p>
diff --git a/packages/svelte/tests/migrate/samples/props-on-script-tag-line/output.svelte b/packages/svelte/tests/migrate/samples/props-on-script-tag-line/output.svelte
new file mode 100644
index 0000000000..add32531da
--- /dev/null
+++ b/packages/svelte/tests/migrate/samples/props-on-script-tag-line/output.svelte
@@ -0,0 +1,3 @@
+<script>
+	let { answer } = $props();</script>
+<p>{answer}</p>
\ No newline at end of file