Actually initialize __main__.__loader__ with loader instances, not the corresponding type objects

Lib/test/test_cmd_line_script.py

@@ -37,7 +37,8 @@ f()
 assertEqual(result, ['Top level assignment', 'Lower level reference'])
 # Check population of magic variables
 assertEqual(__name__, '__main__')
-_loader = __loader__ if isinstance(__loader__, type) else type(__loader__)
+from importlib.machinery import BuiltinImporter
+_loader = __loader__ if __loader__ is BuiltinImporter else type(__loader__)
 print('__loader__==%a' % _loader)
 print('__file__==%a' % __file__)
 assertEqual(__cached__, None)

Python/pythonrun.c

@@ -1355,11 +1355,15 @@ set_main_loader(PyObject *d, const char *filename, const char *loader_name)
 {
     PyInterpreterState *interp;
     PyThreadState *tstate;
-    PyObject *loader;
+    PyObject *loader_type, *loader;
     /* Get current thread state and interpreter pointer */
     tstate = PyThreadState_GET();
     interp = tstate->interp;
-    loader = PyObject_GetAttrString(interp->importlib, loader_name);
+    loader_type = PyObject_GetAttrString(interp->importlib, loader_name);
+    if (loader_type == NULL) {
+        return -1;
+    }
+    loader = PyObject_CallFunction(loader_type, "ss", "__main__", filename);
     if (loader == NULL ||
         (PyDict_SetItemString(d, "__loader__", loader) < 0)) {
         return -1;