NodeResolver: Reload the closest package.json to an asset if it's a package entry

packages/core/integration-tests/test/integration/scope-hoisting/es6/side-effects-package-redirect-down/index.js

@@ -0,0 +1,3 @@
+import {one} from 'foo/bar'
+
+output = one;

packages/core/integration-tests/test/integration/scope-hoisting/es6/side-effects-package-redirect-up/index.js

@@ -0,0 +1,3 @@
+import {one} from 'foo/bar'
+
+output = one;

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

@@ -2319,6 +2319,40 @@ describe('scope hoisting', function () {
       assert.equal(await output, 42);
     });
 
+    describe("considers an asset's closest package.json for sideEffects, not the package through which it found the asset", () => {
+      it('handles redirects up the tree', async () => {
+        let b = await bundle(
+          path.join(
+            __dirname,
+            '/integration/scope-hoisting/es6/side-effects-package-redirect-up/index.js',
+          ),
+        );
+
+        let result = await run(b);
+        assert.strictEqual(result, 1);
+
+        let bar = findAsset(b, 'real-bar.js');
+        assert(bar);
+        assert.strictEqual(bar.sideEffects, false);
+      });
+
+      it('handles redirects down the tree', async () => {
+        let b = await bundle(
+          path.join(
+            __dirname,
+            '/integration/scope-hoisting/es6/side-effects-package-redirect-down/index.js',
+          ),
+        );
+
+        let result = await run(b);
+        assert.strictEqual(result, 1);
+
+        let bar = findAsset(b, 'real-bar.js');
+        assert(bar);
+        assert.strictEqual(bar.sideEffects, false);
+      });
+    });
+
     describe('correctly updates used symbols on changes', () => {
       it('dependency symbols change', async function () {
         let testDir = path.join(

packages/utils/node-resolver-core/src/NodeResolver.js

@@ -997,7 +997,15 @@ export default class NodeResolver {
     }
 
     if (found) {
-      return {path: found, pkg};
+      return {
+        path: found,
+        // If this package.json isn't a sibling of found, it's possible pkg is not the
+        // closest package.json to the resolved file. Reload it instead.
+        pkg:
+          pkg == null || pkg?.pkgdir !== path.dirname(found)
+            ? await this.findPackage(found, ctx)
+            : pkg,
+      };
     }
 
     return null;