Add test that fails

src/compiler/program.ts

@@ -936,6 +936,7 @@ namespace ts {
             getOptionsDiagnostics,
             getGlobalDiagnostics,
             getSemanticDiagnostics,
+            getCachedSemanticDiagnostics,
             getSuggestionDiagnostics,
             getDeclarationDiagnostics,
             getBindAndCheckDiagnostics,
@@ -1656,6 +1657,12 @@ namespace ts {
             return getDiagnosticsHelper(sourceFile, getSemanticDiagnosticsForFile, cancellationToken);
         }
 
+        function getCachedSemanticDiagnostics(sourceFile?: SourceFile): readonly Diagnostic[] | undefined {
+           return sourceFile
+                ? cachedBindAndCheckDiagnosticsForFile.perFile?.get(sourceFile.path)
+                : cachedBindAndCheckDiagnosticsForFile.allDiagnostics;
+        }
+
         function getBindAndCheckDiagnostics(sourceFile: SourceFile, cancellationToken?: CancellationToken): readonly Diagnostic[] {
             return getBindAndCheckDiagnosticsForFile(sourceFile, cancellationToken);
         }

src/compiler/types.ts

@@ -3762,6 +3762,8 @@ namespace ts {
         /* @internal */ getDiagnosticsProducingTypeChecker(): TypeChecker;
         /* @internal */ dropDiagnosticsProducingTypeChecker(): void;
 
+        /* @internal */ getCachedSemanticDiagnostics(sourceFile?: SourceFile): readonly Diagnostic[] | undefined;
+
         /* @internal */ getClassifiableNames(): Set<__String>;
 
         getTypeCatalog(): readonly Type[];

src/testRunner/unittests/tscWatch/watchApi.ts

@@ -123,4 +123,36 @@ namespace ts.tscWatch {
             checkProgramActualFiles(watch.getProgram().getProgram(), [mainFile.path, otherFile.path, libFile.path]);
         });
     });
+
+    describe("unittests:: tsc-watch:: watchAPI:: when watchHost uses createSemanticDiagnosticsBuilderProgram", () => {
+        it("verifies that noEmit is handled on createSemanticDiagnosticsBuilderProgram and typechecking happens only on affected files", () => {
+            const config: File = {
+                path: `${projectRoot}/tsconfig.json`,
+                content: "{}"
+            };
+            const mainFile: File = {
+                path: `${projectRoot}/main.ts`,
+                content: "export const x = 10;"
+            };
+            const otherFile: File = {
+                path: `${projectRoot}/other.ts`,
+                content: "export const y = 10;"
+            };
+            const sys = createWatchedSystem([config, mainFile, otherFile, libFile]);
+            const watchCompilerHost = createWatchCompilerHost(
+                config.path,
+                { noEmit: true },
+                sys,
+                createSemanticDiagnosticsBuilderProgram
+            );
+            const watch = createWatchProgram(watchCompilerHost);
+            checkProgramActualFiles(watch.getProgram().getProgram(), [mainFile.path, otherFile.path, libFile.path]);
+            sys.appendFile(mainFile.path, "\n// SomeComment");
+            sys.runQueuedTimeoutCallbacks();
+            const program = watch.getProgram().getProgram();
+            assert.deepEqual(program.getCachedSemanticDiagnostics(program.getSourceFile(mainFile.path)), []);
+            // Should not retrieve diagnostics for other file thats not changed
+            assert.deepEqual(program.getCachedSemanticDiagnostics(program.getSourceFile(otherFile.path)), /*expected*/ undefined);
+        });
+    });
 }