Don't split ExportDefaultDeclaration in Class visitor

packages/babel-plugin-proposal-decorators/src/transformer.js

@@ -162,6 +162,8 @@ export default ({ automaticParentheses }) => {
   }
 
   function transformClass(path, file, automaticParentheses) {
+    const isDeclaration = path.node.id && path.isDeclaration();
+
     path.node.type = "ClassDeclaration";
     if (!path.node.id) path.node.id = path.scope.generateUidIdentifier("class");
 
@@ -172,44 +174,30 @@ export default ({ automaticParentheses }) => {
 
     insertInitializeInstanceElements(path, initializeId);
 
-    return template.expression.ast`
-    ${file.addHelper("decorate")}(
-      ${classDecorators || t.nullLiteral()},
-      function (${initializeId}) {
-        ${path.node}
-        return { F: ${t.cloneNode(path.node.id)}, d: ${definitions} };
-      }
-    )
-  `;
+    const expr = template.expression.ast`
+      ${file.addHelper("decorate")}(
+        ${classDecorators || t.nullLiteral()},
+        function (${initializeId}) {
+          ${path.node}
+          return { F: ${t.cloneNode(path.node.id)}, d: ${definitions} };
+        }
+      )
+    `;
+
+    return isDeclaration ? template.ast`let ${path.node.id} = ${expr}` : expr;
   }
 
   return {
-    ClassDeclaration(path) {
-      if (!hasDecorators(path)) return;
-
-      if (path.parentPath.isExportDefaultDeclaration()) {
-        if (!path.node.id) {
-          t.toExpression(path.node);
-          path.replaceWith(
-            transformClass(path, this.file, automaticParentheses),
-          );
-          return;
-        }
+    ExportDefaultDeclaration(path) {
+      let decl = path.get("declaration");
+      if (!decl.isClassDeclaration() || !hasDecorators(decl)) return;
 
-        path = splitExportDeclaration(path.parentPath);
-      }
+      if (decl.node.id) decl = splitExportDeclaration(path);
 
-      path.replaceWith(
-        t.variableDeclaration("let", [
-          t.variableDeclarator(
-            t.cloneNode(path.node.id),
-            transformClass(path, this.file),
-          ),
-        ]),
-      );
+      decl.replaceWith(transformClass(decl, this.file, automaticParentheses));
     },
 
-    ClassExpression(path) {
+    Class(path) {
       if (hasDecorators(path)) {
         path.replaceWith(transformClass(path, this.file, automaticParentheses));
       }

packages/babel-plugin-proposal-decorators/test/fixtures/transformation/export-default-named/input.mjs

@@ -0,0 +1,2 @@
+@dec()
+export default class Foo {}

packages/babel-plugin-proposal-decorators/test/fixtures/transformation/export-default-named/output.mjs

@@ -0,0 +1,14 @@
+let Foo = babelHelpers.decorate([dec()], function (_initialize) {
+  class Foo {
+    constructor() {
+      _initialize(this);
+    }
+
+  }
+
+  return {
+    F: Foo,
+    d: []
+  };
+});
+export { Foo as default };