fix corner cases with export default (#4673)

lib/parse.js

@@ -1351,13 +1351,46 @@ function parse($TEXT, options) {
         }
         if (is("keyword", "default")) {
             next();
-            var body = expression();
-            semicolon();
+            var start = S.token;
+            var body = export_default_decl();
+            if (body) {
+                body.start = start;
+                body.end = prev();
+            } else {
+                body = expression();
+                semicolon();
+            }
             return new AST_ExportDefault({ body: body });
         }
         return new AST_ExportDeclaration({ body: export_decl() });
     }
 
+    function maybe_named(def, exp) {
+        var node = function_(exp);
+        if (node.name) {
+            node = new def(node);
+            node.name = new AST_SymbolDefun(node.name);
+        }
+        return node;
+    }
+
+    function export_default_decl() {
+        switch (S.token.value) {
+          case "async":
+            if (!is_token(peek(), "keyword", "function")) return;
+            next();
+            next();
+            if (!is("operator", "*")) return maybe_named(AST_AsyncDefun, AST_AsyncFunction);
+            next();
+            return maybe_named(AST_AsyncGeneratorDefun, AST_AsyncGeneratorFunction);
+          case "function":
+            next();
+            if (!is("operator", "*")) return maybe_named(AST_Defun, AST_Function);
+            next();
+            return maybe_named(AST_GeneratorDefun, AST_GeneratorFunction);
+        }
+    }
+
     var export_decl = embed_tokens(function() {
         switch (S.token.value) {
           case "async":