Use correct file names when writing sourcemaps for multiple outputs using the "dir" option

src/Chunk.ts

@@ -509,6 +509,7 @@ export default class Chunk {
 	) {
 		timeStart('render format', 3);
 
+		const chunkId = this.id!;
 		const format = options.format as string;
 		const finalise = finalisers[format];
 		if (options.dynamicImportFunction && format !== 'es') {
@@ -602,12 +603,12 @@ export default class Chunk {
 
 				let file: string;
 				if (options.file) file = resolve(options.sourcemapFile || options.file);
-				else if (options.dir) file = resolve(options.dir, this.id!);
-				else file = resolve(this.id!);
+				else if (options.dir) file = resolve(options.dir, chunkId);
+				else file = resolve(chunkId);
 
 				const decodedMap = magicString.generateDecodedMap({});
 				map = collapseSourcemaps(
-					this,
+					this.graph,
 					file,
 					decodedMap,
 					this.usedModules,

src/utils/collapseSourcemaps.ts

@@ -1,5 +1,4 @@
 import { DecodedSourceMap, SourceMap } from 'magic-string';
-import Chunk from '../Chunk';
 import Graph from '../Graph';
 import Module from '../Module';
 import {
@@ -200,14 +199,14 @@ function getCollapsedSourcemap(
 }
 
 export function collapseSourcemaps(
-	bundle: Chunk,
+	graph: Graph,
 	file: string,
 	map: DecodedSourceMap,
 	modules: Module[],
 	bundleSourcemapChain: DecodedSourceMapOrMissing[],
 	excludeContent: boolean | undefined
 ) {
-	const linkMap = getLinkMap(bundle.graph);
+	const linkMap = getLinkMap(graph);
 	const moduleSources = modules
 		.filter(module => !module.excludeFromSourcemap)
 		.map(module =>

test/cli/samples/config-multiple-source-maps/_config.js

@@ -0,0 +1,4 @@
+module.exports = {
+	description: 'correctly generates sourcemaps for multiple outputs',
+	command: 'rollup -c'
+};

test/cli/samples/config-multiple-source-maps/main.js

@@ -0,0 +1 @@
+assert.equal( ANSWER, 42 );

test/cli/samples/config-multiple-source-maps/rollup.config.js

@@ -0,0 +1,17 @@
+export default {
+	input: 'main.js',
+	output: [
+		{
+			format: 'cjs',
+			dir: '_actual',
+			entryFileNames: '[name]-[format].js',
+			sourcemap: true,
+		},
+		{
+			format: 'es',
+			dir: '_actual',
+			entryFileNames: '[name]-[format].js',
+			sourcemap: true
+		}
+	]
+};