From b8453076989aac5175f7322f468338b60293eaeb Mon Sep 17 00:00:00 2001
From: Bjorn Lu <34116392+bluwy@users.noreply.github.com>
Date: Thu, 23 Sep 2021 01:21:40 +0800
Subject: [PATCH] [fix] handle preprocess with empty sourcemaps (#6757)

---
 src/compiler/preprocess/index.ts                      | 11 +++++++----
 test/preprocess/samples/empty-sourcemap/_config.js    |  7 +++++++
 test/preprocess/samples/empty-sourcemap/input.svelte  |  7 +++++++
 test/preprocess/samples/empty-sourcemap/output.svelte |  7 +++++++
 4 files changed, 28 insertions(+), 4 deletions(-)
 create mode 100644 test/preprocess/samples/empty-sourcemap/_config.js
 create mode 100644 test/preprocess/samples/empty-sourcemap/input.svelte
 create mode 100644 test/preprocess/samples/empty-sourcemap/output.svelte

diff --git a/src/compiler/preprocess/index.ts b/src/compiler/preprocess/index.ts
index 10771603d7..c2a8e3b7d2 100644
--- a/src/compiler/preprocess/index.ts
+++ b/src/compiler/preprocess/index.ts
@@ -79,10 +79,13 @@ function processed_content_to_code(processed: Processed, location: SourceLocatio
 	if (processed.map) {
 		decoded_map = decode_map(processed);
 
-		// offset only segments pointing at original component source
-		const source_index = decoded_map.sources.indexOf(file_basename);
-		if (source_index !== -1) {
-			sourcemap_add_offset(decoded_map, location, source_index);
+		// decoded map may not have sources for empty maps like `{ mappings: '' }`
+		if (decoded_map.sources) {
+			// offset only segments pointing at original component source
+			const source_index = decoded_map.sources.indexOf(file_basename);
+			if (source_index !== -1) {
+				sourcemap_add_offset(decoded_map, location, source_index);
+			}
 		}
 	}
 
diff --git a/test/preprocess/samples/empty-sourcemap/_config.js b/test/preprocess/samples/empty-sourcemap/_config.js
new file mode 100644
index 0000000000..941661ce01
--- /dev/null
+++ b/test/preprocess/samples/empty-sourcemap/_config.js
@@ -0,0 +1,7 @@
+export default {
+	preprocess: {
+		style: ({ content }) => {
+			return { code: content, map: { mappings: '' } };
+		}
+	}
+};
diff --git a/test/preprocess/samples/empty-sourcemap/input.svelte b/test/preprocess/samples/empty-sourcemap/input.svelte
new file mode 100644
index 0000000000..d2380c00cb
--- /dev/null
+++ b/test/preprocess/samples/empty-sourcemap/input.svelte
@@ -0,0 +1,7 @@
+<div class="foo">bar</div>
+
+<style>
+	.foo {
+		color: red;
+	}
+</style>
\ No newline at end of file
diff --git a/test/preprocess/samples/empty-sourcemap/output.svelte b/test/preprocess/samples/empty-sourcemap/output.svelte
new file mode 100644
index 0000000000..d2380c00cb
--- /dev/null
+++ b/test/preprocess/samples/empty-sourcemap/output.svelte
@@ -0,0 +1,7 @@
+<div class="foo">bar</div>
+
+<style>
+	.foo {
+		color: red;
+	}
+</style>
\ No newline at end of file