Handle the fact that noResolveResolution resolution is reused

src/compiler/resolutionCache.ts

@@ -568,7 +568,7 @@ namespace ts {
             }
             else {
                 resolution.refCount = 1;
-                Debug.assert(resolution.files === undefined);
+                Debug.assert(length(resolution.files) === 0); // This resolution shouldnt be referenced by any file yet
                 if (isExternalModuleNameRelative(name)) {
                     watchFailedLookupLocationOfResolution(resolution);
                 }

src/testRunner/unittests/tsserver/approximateSemanticOnlyServer.ts

@@ -213,5 +213,23 @@ function fooB() { }`
             assert.isFalse(project.dirty);
             checkProjectActualFiles(project, [libFile.path, file1.path, file2.path, file3.path, something.path]);
         });
+
+        it("should not crash when external module name resolution is reused", () => {
+            const { session, file1, file2, file3 } = setup();
+            const service = session.getProjectService();
+            openFilesForSession([file1], session);
+            checkNumberOfProjects(service, { inferredProjects: 1 });
+            const project = service.inferredProjects[0];
+            checkProjectActualFiles(project, [libFile.path, file1.path, file2.path]);
+
+            // Close the file that contains non relative external module name and open some file that doesnt have non relative external module import
+            closeFilesForSession([file1], session);
+            openFilesForSession([file3], session);
+            checkProjectActualFiles(project, [libFile.path, file3.path]);
+
+            // Open file with non relative external module name
+            openFilesForSession([file2], session);
+            checkProjectActualFiles(project, [libFile.path, file2.path, file3.path]);
+        });
     });
 }