From 4ca2af41df436ef97cd1953eb67c2c4922a8257b Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Fri, 23 Jul 2021 18:44:26 +0200
Subject: [PATCH] [fix] add filename to combined source map if needed (#6089)

---
 src/compiler/utils/mapped_code.ts                      | 5 +++++
 test/sourcemaps/samples/preprocessed-no-map/_config.js | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/compiler/utils/mapped_code.ts b/src/compiler/utils/mapped_code.ts
index 12a3ce1d16..58f44d7b8c 100644
--- a/src/compiler/utils/mapped_code.ts
+++ b/src/compiler/utils/mapped_code.ts
@@ -251,6 +251,11 @@ export function combine_sourcemaps(
 
 	if (!map.file) delete map.file; // skip optional field `file`
 
+	// When source maps are combined and the leading map is empty, sources is not set.
+	// Add the filename to the empty array in this case. 
+	// Further improvements to remapping may help address this as well https://github.com/ampproject/remapping/issues/116
+	if (!map.sources.length) map.sources = [filename];
+
 	return map;
 }
 
diff --git a/test/sourcemaps/samples/preprocessed-no-map/_config.js b/test/sourcemaps/samples/preprocessed-no-map/_config.js
index 9888b56ddf..78b0ee626f 100644
--- a/test/sourcemaps/samples/preprocessed-no-map/_config.js
+++ b/test/sourcemaps/samples/preprocessed-no-map/_config.js
@@ -1,5 +1,5 @@
 export default {
-	css_map_sources: [],
+	css_map_sources: ['input.svelte'],
 	preprocess: [
 		{
 			style: ({ content }) => {