Fix getTypeAtLocation for as const to not issue a diagnostic (#36741)

* Fix getTypeAtLocation for `as const` to not issue a diagnostic

* use existing helpers for checks

* Fix lint

Author: weswigham

src/compiler/checker.ts

@@ -11337,6 +11337,11 @@ namespace ts {
         function getTypeFromTypeReference(node: TypeReferenceType): Type {
             const links = getNodeLinks(node);
             if (!links.resolvedType) {
+                // handle LS queries on the `const` in `x as const` by resolving to the type of `x`
+                if (isConstTypeReference(node) && isAssertionExpression(node.parent)) {
+                    links.resolvedSymbol = unknownSymbol;
+                    return links.resolvedType = checkExpressionCached(node.parent.expression);
+                }
                 let symbol: Symbol | undefined;
                 let type: Type | undefined;
                 const meaning = SymbolFlags.Type;

src/testRunner/unittests/programApi.ts

@@ -180,6 +180,17 @@ namespace ts {
     });
 
     describe("unittests:: programApi:: Program.getDiagnosticsProducingTypeChecker / Program.getSemanticDiagnostics", () => {
+        it("does not produce errors on `as const` it would not normally produce on the command line", () => {
+            const main = new documents.TextDocument("/main.ts", "0 as const");
+
+            const fs = vfs.createFromFileSystem(Harness.IO, /*ignoreCase*/ false, { documents: [main], cwd: "/" });
+            const program = createProgram(["/main.ts"], {}, new fakes.CompilerHost(fs, { newLine: NewLineKind.LineFeed }));
+            const typeChecker = program.getDiagnosticsProducingTypeChecker();
+            const sourceFile = program.getSourceFile("main.ts")!;
+            typeChecker.getTypeAtLocation(((sourceFile.statements[0] as ExpressionStatement).expression as AsExpression).type);
+            const diag = program.getSemanticDiagnostics();
+            assert.isEmpty(diag);
+        });
         it("getSymbolAtLocation does not cause additional error to be added on module resolution failure", () => {
             const main = new documents.TextDocument("/main.ts", "import \"./module\";");
             const mod = new documents.TextDocument("/module.d.ts", "declare const foo: any;");