Resolve re-exports when looking for tslib helpers

Author: andrewbranch

src/compiler/checker.ts

@@ -46998,7 +46998,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                                 if (requestedExternalEmitHelperNames.has(name)) continue;
                                 requestedExternalEmitHelperNames.add(name);
 
-                                const symbol = getSymbol(helpersModule.exports!, escapeLeadingUnderscores(name), SymbolFlags.Value);
+                                const symbol = getSymbol(getExportsOfModule(helpersModule), escapeLeadingUnderscores(name), SymbolFlags.Value);
                                 if (!symbol) {
                                     error(location, Diagnostics.This_syntax_requires_an_imported_helper_named_1_which_does_not_exist_in_0_Consider_upgrading_your_version_of_0, externalHelpersModuleNameText, name);
                                 }

tests/cases/compiler/tslibReExportHelpers.ts

@@ -0,0 +1,29 @@
+// @module: nodenext
+// @experimentalDecorators: true
+// @importHelpers: true
+
+// @Filename: /node_modules/tslib/index.d.ts
+export declare function __decorate(...args: any[]): any;
+
+// @Filename: /node_modules/tslib/index.d.mts
+export * from "./index.js";
+
+// @Filename: /node_modules/tslib/package.json
+{
+    "name": "tslib",
+    "version": "1.0.0",
+    "types": "index.d.ts",
+    "exports": {
+        ".": {
+            "types": {
+                "import": "./index.d.mts",
+                "default": "./index.d.ts"
+            }
+        }
+    }
+}
+
+// @Filename: /index.mts
+declare var decorator: any;
+@decorator
+export class Foo {}