Add test for sourcemap offset

src/compiler/preprocess/index.ts

@@ -191,9 +191,11 @@ function get_replacement(
 			decoded_map = decoded_sourcemap_from_generator(decoded_map);
 		}
 		// offset only segments pointing at original component source
-		const source_index = decoded_map.sources.findIndex(source => source === file_basename);
+		const source_index = decoded_map.sources.indexOf(file_basename);
+		if (source_index !== -1) {
 			sourcemap_add_offset(decoded_map, get_location(offset + prefix.length), source_index);
 		}
+	}
 	const processed_with_map = StringWithSourcemap.from_processed(processed.code, decoded_map);
 
 	// Surround the processed code with the prefix and suffix, retaining valid sourcemappings

src/compiler/utils/string_with_sourcemap.ts

@@ -15,7 +15,7 @@ function last_line_length(s: string) {
 export function sourcemap_add_offset(
 	map: DecodedSourceMap, offset: SourceLocation, source_index: number
 ) {
-	if (map.mappings.length == 0 || source_index < 0) return;
+	if (map.mappings.length == 0) return;
 	// shift lines
 	for (let line = 0; line < map.mappings.length; line++) {
 		const segment_list = map.mappings[line];

test/sourcemaps/samples/concat/_config.js

@@ -0,0 +1,15 @@
+import MagicString from 'magic-string';
+import { magic_string_preprocessor_result } from '../../helpers';
+
+export default {
+	js_map_sources: ['input.svelte'],
+	preprocess: [
+		{
+			script: ({ content }) => {
+				const src = new MagicString(content);
+				src.prepend('console.log("Injected first line");\n');
+				return magic_string_preprocessor_result('input.svelte', src);
+			}
+		}
+	]
+};

test/sourcemaps/samples/concat/input.svelte

@@ -0,0 +1,3 @@
+<script>console.log('Target')</script>
+
+<h1>Hello</h1>

test/sourcemaps/samples/concat/test.js

@@ -0,0 +1,9 @@
+import { assert_mapped } from '../../helpers';
+
+export function test({ input, preprocessed }) {
+	assert_mapped({
+    code: 'Target',
+		input: input.locate,
+		preprocessed
+	});
+}