Compile decorators properly when googmodule is false and transformTypesToClosure is true

package.json

@@ -21,7 +21,7 @@
     "@types/source-map-support": "^0.5.3",
     "diff-match-patch": "^1.0.5",
     "glob": "8.0.1",
-    "google-closure-compiler": "^20230411.0.0",
+    "google-closure-compiler": "^20230502.0.0",
     "jasmine": "^4.1.0",
     "jasmine-node": "^3.0.0",
     "source-map": "^0.7.3",

src/fix_downleveled_decorators.ts

@@ -0,0 +1,60 @@
+import * as ts from "typescript";
+
+/**
+ * This is an attempt to fix the downleveled decorators so Closure doesn't have
+ * trouble with them. The problem is that when `experimentalDecorators` is
+ * enabled, we TSC ends up converting a class decorator like this:
+ *
+ * @classDecorator
+ * export class Person { ... }
+ *
+ * to this:
+ *
+ * let Person = class Person { ... };
+ *
+ * as well as some calls to __decorate(Person, ...)
+ *
+ * The problem is that this causes Closure Compiler to fail with this error:
+ * ERROR - [JSC_CANNOT_CONVERT_YET] Transpilation of 'Classes with possible name shadowing' is not yet implemented.
+ * 21| let Person = class Person {
+ *                  ^^^^^^^^^^^^^^
+ *
+ * As previously mentioned, this transformer is an _attempt_, it doesn't actually work it seems.
+ * I'm not sure if tsickle can even affect this at this point in the process or not.
+ */
+export function fixDownleveledDecorators() {
+  return (context: ts.TransformationContext): ts.Transformer<ts.SourceFile> => {
+    return (sourceFile: ts.SourceFile) => {
+      function visit(node: ts.Node): ts.Node {
+        // Check if the node is a VariableDeclaration
+        if (ts.isVariableDeclaration(node)) {
+          // Check if it's a variable declaration of a class expression
+          if (node.initializer && ts.isClassExpression(node.initializer)) {
+            const className = node.name;
+            const classNameAsString = className.getText();
+
+            const typeParameter = ts.factory.createTypeParameterDeclaration(
+              undefined,
+              classNameAsString,
+              undefined,
+              undefined
+            );
+
+            const classDeclaration = ts.factory.createClassDeclaration(
+              undefined,
+              undefined,
+              [typeParameter],
+              [],
+              node.initializer.members
+            );
+
+            // Replace the variable declaration with the class declaration
+            ts.factory.updateSourceFile(sourceFile, [classDeclaration]);
+          }
+        }
+        return ts.visitEachChild(node, visit, context);
+      }
+      return ts.visitEachChild(sourceFile, visit, context);
+    };
+  };
+}

src/tsickle.ts

@@ -24,6 +24,7 @@ import {FileSummary, SummaryGenerationProcessorHost} from './summary';
 import {isDtsFileName} from './transformer_util';
 import * as tsmes from './ts_migration_exports_shim';
 import {makeTsickleDeclarationMarkerTransformerFactory} from './tsickle_declaration_marker';
+import {fixDownleveledDecorators} from './fix_downleveled_decorators';
 
 // Exported for users as a default impl of pathToModuleName.
 export {pathToModuleName} from './cli_support';
@@ -253,6 +254,7 @@ export function emit(
       transformDecoratorsOutputForClosurePropertyRenaming(
           tsickleDiagnostics, false));
     tsTransformers.after!.push(transformDecoratorJsdoc());
+    tsTransformers.after!.push(fixDownleveledDecorators())
   }
   if (host.addDtsClutzAliases) {
     tsTransformers.afterDeclarations!.push(