Fix TypeErrors from decorated class emit.

src/compiler/checker.ts

@@ -16854,6 +16854,11 @@ namespace ts {
 
         function isValueAliasDeclaration(node: Node): boolean {
             node = getSourceTreeNode(node);
+            if (node === undefined) {
+                // A synthesized node comes from an emit transformation and is always a value.
+                return true;
+            }
+
             switch (node.kind) {
                 case SyntaxKind.ImportEqualsDeclaration:
                 case SyntaxKind.ImportClause:

src/compiler/transformers/module/module.ts

@@ -735,15 +735,7 @@ namespace ts {
                 const classDecl = original as ClassDeclaration;
                 if (classDecl.name) {
                     const statements = [node];
-                    // Avoid emitting a default because a decorated default-exported class will have been rewritten in the TS transformer to
-                    // a decorator assignment (`foo = __decorate(...)`) followed by a separate default export declaration (`export default foo`).
-                    // We will eventually take care of that default export assignment when we transform the generated default export declaration.
-                    if (hasModifier(classDecl, ModifierFlags.Export) && !hasModifier(classDecl, ModifierFlags.Default)) {
-                        addExportMemberAssignment(statements, classDecl)
-                    }
-
                     addExportMemberAssignments(statements, classDecl.name);
-
                     return statements;
                 }
             }

src/compiler/transformers/ts.ts

@@ -420,6 +420,26 @@ namespace ts {
             }
         }
 
+        /**
+         * Tests whether we should emit a __decorate call for a class declaration.
+         */
+        function shouldEmitDecorateCallForClass(node: ClassDeclaration) {
+            if (node.decorators && node.decorators.length > 0) {
+                return true;
+            }
+
+            const constructor = getFirstConstructorWithBody(node);
+            if (constructor) {
+                for (const parameter of constructor.parameters) {
+                    if (parameter.decorators && parameter.decorators.length > 0) {
+                        return true;
+                    }
+                }
+            }
+
+            return false;
+        }
+
         /**
          * Transforms a class declaration with TypeScript syntax into compatible ES6.
          *
@@ -434,6 +454,7 @@ namespace ts {
         function visitClassDeclaration(node: ClassDeclaration): VisitResult<Statement> {
             const staticProperties = getInitializedProperties(node, /*isStatic*/ true);
             const hasExtendsClause = getClassExtendsHeritageClauseElement(node) !== undefined;
+            const isDecoratedClass = shouldEmitDecorateCallForClass(node);
             let decoratedClassAlias: Identifier;
 
             // emit name if
@@ -446,7 +467,7 @@ namespace ts {
             }
 
             const statements: Statement[] = [];
-            if (!node.decorators) {
+            if (!isDecoratedClass) {
                 //  ${modifiers} class ${name} ${heritageClauses} {
                 //      ${members}
                 //  }
@@ -491,7 +512,7 @@ namespace ts {
             if (isNamespaceExport(node)) {
                 addExportMemberAssignment(statements, node);
             }
-            else if (node.decorators) {
+            else if (isDecoratedClass) {
                 if (isDefaultExternalModuleExport(node)) {
                     statements.push(createExportDefault(getLocalName(node)));
                 }
@@ -644,19 +665,11 @@ namespace ts {
                     /*location*/ location);
             }
 
-            // When emitting as a *default* export, we'll add a subsequent `export default` statement,
-            // so we should only be creating a local binding without any modifiers.
-            // Otherwise, we need preserve and visit all the modifiers.
-            const bindingModifiers =
-                isDefaultExternalModuleExport(node)
-                    ? undefined
-                    : visitNodes(node.modifiers, visitor, isModifier);
-
             //  let ${name} = ${classExpression};
             addNode(statements,
                 setOriginalNode(
                     createVariableStatement(
-                        bindingModifiers,
+                        /*modifiers*/ undefined,
                         createLetDeclarationList([
                             createVariableDeclaration(
                                 getDeclarationName(node, /*allowComments*/ true),

tests/baselines/reference/decoratorOnClass2.es6.js

@@ -14,9 +14,10 @@ var __decorate = (this && this.__decorate) || function (decorators, target, key,
     else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
     return c > 3 && r && Object.defineProperty(target, key, r), r;
 };
-export let C = class C {
+let C = class C {
 };
 C = __decorate([
     dec
 ], C);
+export { C };
 let c = new C();

tests/baselines/reference/decoratorOnClass6.es6.js

@@ -17,11 +17,12 @@ var __decorate = (this && this.__decorate) || function (decorators, target, key,
     return c > 3 && r && Object.defineProperty(target, key, r), r;
 };
 let C_1;
-export let C = C_1 = class C {
+let C = C_1 = class C {
     static x() { return C_1.y; }
 };
 C.y = 1;
 C = C_1 = __decorate([
     dec
 ], C);
+export { C };
 let c = new C();

tests/baselines/reference/decoratorOnClassConstructor2.js

@@ -49,7 +49,6 @@ var C = (function (_super) {
     }
     return C;
 }(_0_ts_1.base));
-exports.C = C;
 C = __decorate([
     __param(0, _0_ts_2.foo)
 ], C);