composeSourceMaps: Keep mappings that share a generated column

packages/metro-source-map/src/__tests__/composeSourceMaps-test.js

@@ -294,6 +294,23 @@ describe('composeSourceMaps', () => {
     expect(mergedMap).toEqual(fixtures['merged_1_2.json']);
   });
 
+  test('keeps each mapping at a generated column mapped more than once', () => {
+    const mergedMap = composeSourceMaps([
+      {version: 3, names: [], sources: ['a.js'], mappings: 'AAAA,KAAK'},
+      {version: 3, names: [], sources: ['b.js'], mappings: 'AAAA,AAAK'},
+    ]);
+    expect(
+      [...new Consumer(mergedMap).generatedMappings()].map(mapping => [
+        mapping.generatedColumn,
+        mapping.originalLine,
+        mapping.originalColumn,
+      ]),
+    ).toEqual([
+      [0, 1, 0],
+      [0, 1, 5],
+    ]);
+  });
+
   test('merges two maps preserving unmapped regions in the first one', () => {
     const mergedMap = composeSourceMaps([
       fixtures['ignore_1.json'],

packages/metro-source-map/src/composeSourceMaps.js

@@ -45,12 +45,23 @@ export default function composeSourceMaps(
     file: consumers[0].file,
   });
 
+  const earlierConsumers = consumers.slice(1);
   consumers[0].eachMapping(mapping => {
-    const original = findOriginalPosition(
-      consumers,
-      mapping.generatedLine,
-      mapping.generatedColumn,
-    );
+    const original =
+      mapping.originalLine == null || mapping.originalColumn == null
+        ? {line: null, column: null, source: null, name: null}
+        : earlierConsumers.length === 0
+          ? {
+              line: mapping.originalLine,
+              column: mapping.originalColumn,
+              source: mapping.source,
+              name: mapping.name,
+            }
+          : findOriginalPosition(
+              earlierConsumers,
+              mapping.originalLine,
+              mapping.originalColumn,
+            );
     generator.addMapping({
       generated: {
         line: mapping.generatedLine,