fix: LS not showing existing diagnostics on file open

integration/lsp/viewengine_spec.ts

@@ -86,6 +86,25 @@ describe('Angular language server', () => {
       },
     });
   });
+
+  it('should show existing diagnostics on external template', async () => {
+    client.sendNotification(lsp.DidOpenTextDocumentNotification.type, {
+      textDocument: {
+        uri: `file://${FOO_TEMPLATE}`,
+        languageId: 'typescript',
+        version: 1,
+        text: `{{ doesnotexist }}`,
+      },
+    });
+    const diagnostics: lsp.Diagnostic[] = await new Promise(resolve => {
+      client.onNotification(
+          lsp.PublishDiagnosticsNotification.type, (params: lsp.PublishDiagnosticsParams) => {
+            resolve(params.diagnostics);
+          });
+    });
+    expect(diagnostics.length).toBe(1);
+    expect(diagnostics[0].message).toContain(`Identifier 'doesnotexist' is not defined.`);
+  });
 });
 
 describe('initialization', () => {

server/src/session.ts

@@ -272,17 +272,9 @@ export class Session {
         // configFileErrors is an empty array even if there's no error, so check length.
         this.connection.console.error(configFileErrors.map(e => e.messageText).join('\n'));
       }
-      if (!configFileName) {
-        // It is not really an error if there is no config file, because the
-        // first call to openClientFile() will create a project for the file if
-        // it does not exist, but the method will not return the config filename.
-        // In subsequent operations, we'll call this.getDefaultProjectForScriptInfo(),
-        // and there we make a second call to openClientFile(). By then, since
-        // the project has already been created, we will receive the config
-        // filename, and we can attach the file to the project it belongs to.
-        return;
-      }
-      const project = this.projectService.findProject(configFileName);
+      const project = configFileName ?
+          this.projectService.findProject(configFileName) :
+          this.projectService.getScriptInfo(filePath)?.containingProjects.find(isConfiguredProject);
       if (!project) {
         this.connection.console.error(`Failed to find project for ${filePath}`);
         return;
@@ -577,3 +569,7 @@ function isAngularProject(project: ts.server.Project, ngCore: string): boolean {
   }
   return false;
 }
+
+function isConfiguredProject(project: ts.server.Project): project is ts.server.ConfiguredProject {
+  return project.projectKind === ts.server.ProjectKind.Configured;
+}