Treat re-exports of '*' from empty files as ESM (#9079)

packages/core/integration-tests/test/integration/scope-hoisting/es6/re-export-all-empty-no-side-effects/index.js

@@ -0,0 +1,3 @@
+import {bar} from 'lib';
+
+output = `foo ${bar}`;

packages/core/integration-tests/test/scope-hoisting.js

@@ -360,6 +360,28 @@ describe('scope hoisting', function () {
       assert.strictEqual(output, '2 4');
     });
 
+    it('supports re-exporting all from an empty module without side effects', async function () {
+      let b = await bundle(
+        path.join(
+          __dirname,
+          '/integration/scope-hoisting/es6/re-export-all-empty-no-side-effects/index.js',
+        ),
+        {
+          mode: 'production',
+        },
+      );
+
+      let output = await run(b);
+      assert.strictEqual(output, 'foo bar');
+
+      let contents = await outputFS.readFile(
+        b.getBundles().find(b => b.getMainEntry().filePath.endsWith('index.js'))
+          .filePath,
+        'utf8',
+      );
+      assert.match(contents, /output="foo bar"/);
+    });
+
     it('supports re-exporting all exports from an external module', async function () {
       let b = await bundle(
         path.join(

packages/transformers/js/src/JSTransformer.js

@@ -813,11 +813,13 @@ export default (new Transformer({
         });
       }
 
-      // Add * symbol if there are CJS exports, no imports/exports at all, or the asset is wrapped.
+      // Add * symbol if there are CJS exports, no imports/exports at all
+      // (and the asset has side effects), or the asset is wrapped.
       // This allows accessing symbols that don't exist without errors in symbol propagation.
       if (
         hoist_result.has_cjs_exports ||
         (!hoist_result.is_esm &&
+          asset.sideEffects &&
           deps.size === 0 &&
           Object.keys(hoist_result.exported_symbols).length === 0) ||
         (hoist_result.should_wrap && !asset.symbols.hasExportSymbol('*'))