[Fix] ExportMap: add caching after parsing for an ambiguous module

src/ExportMap.js

@@ -346,7 +346,11 @@ ExportMap.for = function (context) {
   exportMap = ExportMap.parse(path, content, context);
 
   // ambiguous modules return null
-  if (exportMap == null) return null;
+  if (exportMap == null) {
+    log('ignored path due to ambiguous parse:', path);
+    exportCache.set(cacheKey, null);
+    return null;
+  }
 
   exportMap.mtime = stats.mtime;
 

tests/files/typescript-declare-module.ts

@@ -0,0 +1,3 @@
+declare module "typescript-declare-module-foo" {
+  export const foo: string;
+}

tests/src/core/getExports.js

@@ -104,6 +104,27 @@ describe('ExportMap', function () {
     expect(imports.has('Bar')).to.be.true;
   });
 
+  it('should cache after parsing for an ambiguous module', function () {
+    const context = Object.assign({}, fakeContext, {
+      settings: {
+        'import/extensions': ['.js'],
+        'import/parsers': {
+          '@typescript-eslint/parser': ['.ts', '.tsx'],
+        },
+      },
+    });
+    const source = './typescript-declare-module.ts';
+    const parseSpy = sinon.spy(ExportMap, 'parse');
+
+    expect(ExportMap.get(source, context)).to.be.null;
+
+    ExportMap.get(source, context);
+
+    expect(parseSpy.callCount).to.equal(1);
+
+    parseSpy.restore();
+  });
+
   context('deprecation metadata', function () {
 
     function jsdocTests(parseContext, lineEnding) {