Update logic for shouldSkipSemanticCheck to allow for quering semantic errors for JS files with @ts-check

Author: mhegazy

src/server/session.ts

@@ -25,15 +25,17 @@ namespace ts.server {
         return ((1e9 * seconds) + nanoseconds) / 1000000.0;
     }
 
-    function shouldSkipSemanticCheck(project: Project) {
-        if (project.projectKind === ProjectKind.Inferred || project.projectKind === ProjectKind.External) {
-            return project.isJsOnlyProject();
-        }
-        else {
-            // For configured projects, require that skipLibCheck be set also
-            const options = project.getCompilerOptions();
-            return options.skipLibCheck && !options.checkJs && project.isJsOnlyProject();
+    function shouldSkipSemanticCheck(project: Project, file: NormalizedPath) {
+        // For inferred (e.g. miscellaneous context in VS) and external projects (e.g. VS .csproj project) with only JS files
+        // semantic errors in .d.ts files (e.g. ones added by automatic type acquisition) are not interesting.
+        // We want to avoid the cost of querying for these errors all together, even if 'skipLibCheck' is not set.
+        // We still want to check .js files (e.g. '// @ts-check' or '--checkJs' is set).
+        if ((project.projectKind === ProjectKind.Inferred || project.projectKind === ProjectKind.External) &&
+            project.isJsOnlyProject()) {
+            const scriptInfo = project.getScriptInfoForNormalizedPath(file);
+            return scriptInfo && !scriptInfo.isJavaScript();
         }
+        return false;
     }
 
     interface FileStart {
@@ -489,7 +491,7 @@ namespace ts.server {
         private semanticCheck(file: NormalizedPath, project: Project) {
             try {
                 let diags: Diagnostic[] = [];
-                if (!shouldSkipSemanticCheck(project)) {
+                if (!shouldSkipSemanticCheck(project, file)) {
                     diags = project.getLanguageService().getSemanticDiagnostics(file);
                 }
 
@@ -597,7 +599,7 @@ namespace ts.server {
 
         private getDiagnosticsWorker(args: protocol.FileRequestArgs, isSemantic: boolean, selector: (project: Project, file: string) => Diagnostic[], includeLinePosition: boolean) {
             const { project, file } = this.getFileAndProject(args);
-            if (isSemantic && shouldSkipSemanticCheck(project)) {
+            if (isSemantic && shouldSkipSemanticCheck(project, file)) {
                 return [];
             }
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);