report config errors when config file changed

src/harness/unittests/tsserverProjectSystem.ts

@@ -2452,6 +2452,43 @@ namespace ts.projectSystem {
             openFilesForSession([file], session);
             serverEventManager.checkEventCountOfType("configFileDiag", 1);
         });
+
+        it("are generated when the config file changes", () => {
+            const serverEventManager = new TestServerEventManager();
+            const file = {
+                path: "/a/b/app.ts",
+                content: "let x = 10"
+            };
+            const configFile = {
+                path: "/a/b/tsconfig.json",
+                content: `{
+                    "compilerOptions": {}
+                }`
+            };
+
+            const host = createServerHost([file, configFile]);
+            const session = createSession(host, /*typingsInstaller*/ undefined, serverEventManager.handler);
+            openFilesForSession([file], session);
+            serverEventManager.checkEventCountOfType("configFileDiag", 1);
+
+            configFile.content = `{
+                "compilerOptions": {
+                    "haha": 123
+                }
+            }`;
+            host.reloadFS([file, configFile]);
+            host.triggerFileWatcherCallback(configFile.path);
+            host.runQueuedTimeoutCallbacks();
+            serverEventManager.checkEventCountOfType("configFileDiag", 2);
+
+            configFile.content = `{
+                "compilerOptions": {}
+            }`;
+            host.reloadFS([file, configFile]);
+            host.triggerFileWatcherCallback(configFile.path);
+            host.runQueuedTimeoutCallbacks();
+            serverEventManager.checkEventCountOfType("configFileDiag", 3);
+        });
     });
 
     describe("skipLibCheck", () => {

src/server/editorServices.ts

@@ -520,8 +520,10 @@ namespace ts.server {
         }
 
         private onConfigChangedForConfiguredProject(project: ConfiguredProject) {
-            this.logger.info(`Config file changed: ${project.getConfigFilePath()}`);
-            this.updateConfiguredProject(project);
+            const configFileName = project.getConfigFilePath();
+            this.logger.info(`Config file changed: ${configFileName}`);
+            const configFileErrors = this.updateConfiguredProject(project);
+            this.reportConfigFileDiagnostics(configFileName, configFileErrors, /*triggerFile*/ configFileName);
             this.refreshInferredProjects();
         }
 
@@ -1015,6 +1017,9 @@ namespace ts.server {
                 return;
             }
 
+            // note: the returned "success" is true does not mean the "configFileErrors" is empty.
+            // because we might have tolerated the errors and kept going. So always return the configFileErrors
+            // regardless the "success" here is true or not.
             const { success, projectOptions, configFileErrors } = this.convertConfigFileContentToProjectOptions(project.getConfigFilePath());
             if (!success) {
                 // reset project settings to default
@@ -1026,7 +1031,7 @@ namespace ts.server {
                 project.setCompilerOptions(projectOptions.compilerOptions);
                 if (!project.languageServiceEnabled) {
                     // language service is already disabled
-                    return;
+                    return configFileErrors;
                 }
                 project.disableLanguageService();
                 project.stopWatchingDirectory();
@@ -1038,6 +1043,7 @@ namespace ts.server {
                 this.watchConfigDirectoryForProject(project, projectOptions);
                 this.updateNonInferredProject(project, projectOptions.files, fileNamePropertyReader, projectOptions.compilerOptions, projectOptions.typingOptions, projectOptions.compileOnSave, configFileErrors);
             }
+            return configFileErrors;
         }
 
         createInferredProjectWithRootFileIfNecessary(root: ScriptInfo) {