fix: chain preprocessor sourcemaps with an empty sources[0]

`apply_preprocessor_sourcemap` matches the incoming preprocessor/upstream
sourcemap against `filename` when combining it with svelte's own map. A
map generated via `new MagicString(code).generateMap()` without a
`source` option has `sources: ['']`, which never matches `filename` and
causes the whole chain to be dropped instead of composed — every
mapping through it resolves to `{ source: null, line: null, column: null }`.

Normalize an empty `sources[0]` to `filename` before combining, mirroring
how Vite already treats an empty source as referring to the file being
transformed (`pluginContainer.ts`'s `_getCombinedSourcemap`).

Fixes #18491

.changeset/tame-donkeys-jump.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: chain preprocessor sourcemaps with an empty `sources[0]` instead of dropping them

packages/svelte/src/compiler/utils/mapped_code.js

@@ -311,6 +311,13 @@ function apply_preprocessor_sourcemap(filename, svelte_map, preprocessor_map_inp
 		typeof preprocessor_map_input === 'string'
 			? JSON.parse(preprocessor_map_input)
 			: preprocessor_map_input;
+	// A preprocessor map with a missing/empty `sources[0]` (e.g. from a MagicString transform
+	// created without a `source` option) can't be matched against `filename` during combination,
+	// which silently drops the chain instead of erroring. Normalize it to `filename` first, the
+	// same way Vite treats an empty `sources[0]` as referring to the file being transformed.
+	if (preprocessor_map.sources?.length === 1 && !preprocessor_map.sources[0]) {
+		preprocessor_map.sources = [filename];
+	}
 	const result_map = combine_sourcemaps(filename, [svelte_map, preprocessor_map]);
 	// Svelte expects a SourceMap which includes toUrl and toString. Instead of wrapping our output in a class,
 	// we just tack on the extra properties.

packages/svelte/tests/sourcemaps/samples/sourcemap-empty-source/_config.js

@@ -0,0 +1,29 @@
+import * as fs from 'node:fs';
+import MagicString from 'magic-string';
+import { test } from '../../test';
+
+// Simulates a bundler plugin (e.g. a Vite plugin using `magic-string`) that transforms
+// the Svelte source *before* it reaches `compile()`, and hands its own sourcemap to
+// `compileOptions.sourcemap` — the documented way to let svelte chain an upstream map
+// into its own output map. Crucially, the upstream map is generated *without* a `source`
+// option, exactly like `new MagicString(code).generateMap()` — this yields a sourcemap
+// whose `sources` is `['']`, which previously broke the chain entirely (see #18491)
+// instead of being treated as "this file", causing every mapping through it to resolve
+// to `{ source: null, line: null, column: null }`.
+const input = fs.readFileSync(new URL('./input.svelte', import.meta.url), 'utf-8');
+const src = new MagicString(input);
+src.overwrite(
+	src.original.indexOf('count * 2'),
+	src.original.indexOf('count * 2') + 'count * 2'.length,
+	'count * 2',
+	{
+		storeName: false
+	}
+);
+
+export default test({
+	compileOptions: {
+		sourcemap: src.generateMap({ hires: true })
+	},
+	client: [{ str: 'let doubled' }]
+});

packages/svelte/tests/sourcemaps/samples/sourcemap-empty-source/input.svelte

@@ -0,0 +1,6 @@
+<script>
+	let count = 0;
+	let doubled = count * 2;
+</script>
+
+<button>clicks: {count}</button>