patch export default within sandbox correctly (#5400)

fixes #5399

test/mocha/reduce.js

@@ -434,7 +434,7 @@ describe("test/reduce.js", function() {
             "// }",
         ].join("\n"));
     });
-    it("Should transform `export default` correctly", function() {
+    it("Should transform `export default class` correctly", function() {
         var result = reduce_test(read("test/input/reduce/export_default.js"), {
             compress: false,
             toplevel: true,
@@ -448,4 +448,22 @@ describe("test/reduce.js", function() {
             "// }",
         ].join("\n"));
     });
+    it("Should transform `export default function` correctly", function() {
+        var code = [
+            "for (var k in this)",
+            "    console.log(k);",
+            "export default (function f() {});",
+            "console.log(k);",
+        ].join("\n");
+        var result = reduce_test(code, {
+            mangle: false,
+        });
+        if (result.error) throw result.error;
+        assert.strictEqual(result.code, [
+            "// Can't reproduce test failure",
+            "// minify options: {",
+            '//   "mangle": false',
+            "// }",
+        ].join("\n"));
+    });
 });

test/sandbox.js

@@ -52,12 +52,14 @@ exports.same_stdout = semver.satisfies(process.version, "0.12") ? function(expec
     return typeof expected == typeof actual && strip_func_ids(expected) == strip_func_ids(actual);
 };
 exports.patch_module_statements = function(code) {
-    var count = 0, imports = [];
+    var count = 0, has_default = "", imports = [];
     code = code.replace(/\bexport(?:\s*\{[^{}]*}\s*?(?:$|\n|;)|\s+default\b(?:\s*(\(|\{|class\s*\{|class\s+(?=extends\b)|(?:async\s+)?function\s*(?:\*\s*)?\())?|\b)/g, function(match, header) {
         if (!header) return "";
+        has_default = "var _uglify_export_default_;";
         if (header.length == 1) return "0, " + header;
-        do {
-            var name = "_export_default_" + ++count;
+        var name = "_uglify_export_default_";
+        if (/^class\b/.test(header)) do {
+            name = "_uglify_export_default_" + ++count;
         } while (code.indexOf(name) >= 0);
         return header.slice(0, -1) + " " + name + header.slice(-1);
     }).replace(/\bimport\.meta\b/g, function() {
@@ -76,7 +78,7 @@ exports.patch_module_statements = function(code) {
         return "";
     });
     imports.push("");
-    return imports.join("\n") + code;
+    return has_default + imports.join("\n") + code;
 };
 
 function is_error(result) {