fix(metro-resolver): package exports restricted paths (#1239)

Summary:
This PR deals with restricting internal paths through the most specific subpath pattern edge case from #1236

- [x] - added test for implicit default condition to match Node.js resolution algorithm
- [x] - aligned implementation of `reduceConditionalExport` to account for implicit default condition

Changelog: [Experimental] Fix implicit default condition to be null for subpath patterns (edge case)

Pull Request resolved: #1239

Test Plan: - [x] - all tests pass

Reviewed By: EdmondChuiHW

Differential Revision: D55244268

Pulled By: huntie

fbshipit-source-id: e9cddbad2709a90a0282385a1b59ebc03735f975

packages/metro-resolver/src/PackageExportsResolve.js

@@ -373,7 +373,16 @@ function reduceConditionalExport(
   let reducedValue = subpathValue;
 
   while (reducedValue != null && typeof reducedValue !== 'string') {
-    let match: typeof subpathValue | 'no-match' = 'no-match';
+    let match: typeof subpathValue | 'no-match';
+
+    // when conditions are present and default is not specified
+    // the default condition is implicitly set to null, to allow
+    // for restricting access to unexported internals of a package.
+    if ('default' in reducedValue) {
+      match = 'no-match';
+    } else {
+      match = null;
+    }
 
     for (const conditionName in reducedValue) {
       if (conditionNames.has(conditionName)) {

packages/metro-resolver/src/__tests__/package-exports-test.js

@@ -539,10 +539,6 @@ describe('with package exports resolution enabled', () => {
           'test-pkg/features/foo.js.js',
           '/root/node_modules/test-pkg/src/features/foo.js.js',
         ],
-        [
-          'test-pkg/features/bar/Bar.js',
-          '/root/node_modules/test-pkg/src/features/bar/Bar.js',
-        ],
       ]) {
         expect(Resolver.resolve(baseContext, importSpecifier, null)).toEqual({
           type: 'sourceFile',
@@ -558,7 +554,13 @@ describe('with package exports resolution enabled', () => {
       ).toThrowError();
     });
 
-    test('should use most specific pattern base', () => {
+    test('should use the most specific pattern base - implicit default condition', () => {
+      expect(() =>
+        Resolver.resolve(baseContext, 'test-pkg/features/bar/Bar.js', null),
+      ).toThrowError();
+    });
+
+    test('should use most specific pattern base - custom condition', () => {
       const context = {
         ...baseContext,
         unstable_conditionNames: ['react-native'],