Index: Lib/test/pickletester.py
===================================================================
--- Lib/test/pickletester.py	(revision 82908)
+++ Lib/test/pickletester.py	(working copy)
@@ -65,9 +65,15 @@
     def __eq__(self, other):
         return self.__dict__ == other.__dict__
 
+class D(C):
+    def __init__(self, arg):
+        pass
+
 import __main__
 __main__.C = C
 C.__module__ = "__main__"
+__main__.D = D
+D.__module__ = "__main__"
 
 class myint(int):
     def __init__(self, x):
@@ -425,6 +431,63 @@
     def test_load_from_data2(self):
         self.assertEqual(self._testdata, self.loads(DATA2))
 
+    def test_load_classic_instance(self):
+        # See issue5180.  Test loading 2.x pickles that
+        # contain an instance of old style class.
+        for X, args in [(C, ()), (D, ('x',))]:
+            xname = X.__name__.encode('ascii')
+            # Protocol 0 (text mode pickle):
+            """
+            0: (    MARK
+            1: i        INST       '__main__ X' (MARK at 0)
+            15: p    PUT        0
+            18: (    MARK
+            19: d        DICT       (MARK at 18)
+            20: p    PUT        1
+            23: b    BUILD
+            24: .    STOP
+            """
+            pickle0 = (b"(i__main__\n"
+                       b"X\n"
+                       b"p0\n"
+                       b"(dp1\nb.").replace(b'X', xname)
+            self.assertEqual(X(*args), self.loads(pickle0))
+
+            # Protocol 1 (binary mode pickle)
+            """
+            0: (    MARK
+            1: c        GLOBAL     '__main__ X'
+            15: q        BINPUT     0
+            17: o        OBJ        (MARK at 0)
+            18: q    BINPUT     1
+            20: }    EMPTY_DICT
+            21: q    BINPUT     2
+            23: b    BUILD
+            24: .    STOP
+            """
+            pickle1 = (b'(c__main__\n'
+                       b'X\n'
+                       b'q\x00oq\x01}q\x02b.').replace(b'X', xname)
+            self.assertEqual(X(*args), self.loads(pickle1))
+
+            # Protocol 2 (pickle2 = b'\x80\x02' + pickle1)
+            """
+            0: \x80 PROTO      2
+            2: (    MARK
+            3: c        GLOBAL     '__main__ Bug'
+            17: q        BINPUT     0
+            19: o        OBJ        (MARK at 2)
+            20: q    BINPUT     1
+            22: }    EMPTY_DICT
+            23: q    BINPUT     2
+            25: b    BUILD
+            26: .    STOP
+            """
+            pickle2 = (b'\x80\x02(c__main__\n'
+                       b'X\n'
+                       b'q\x00oq\x01}q\x02b.').replace(b'X', xname)
+            self.assertEqual(X(*args), self.loads(pickle2))
+
     # There are gratuitous differences between pickles produced by
     # pickle and cPickle, largely because cPickle starts PUT indices at
     # 1 and pickle starts them at 0.  See XXX comment in cPickle's put2() --
Index: Lib/pickle.py
===================================================================
--- Lib/pickle.py	(revision 82908)
+++ Lib/pickle.py	(working copy)
@@ -1038,8 +1038,7 @@
         if (not args and
                 isinstance(klass, type) and
                 not hasattr(klass, "__getinitargs__")):
-            value = _EmptyClass()
-            value.__class__ = klass
+            value = klass.__new__(klass)
             instantiated = True
         if not instantiated:
             try:
@@ -1239,15 +1238,8 @@
         raise _Stop(value)
     dispatch[STOP[0]] = load_stop
 
-# Helper class for load_inst/load_obj
+# Encode/decode longs.
 
-class _EmptyClass:
-    pass
-
-# Encode/decode longs in linear time.
-
-import binascii as _binascii
-
 def encode_long(x):
     r"""Encode a long to a two's complement little-endian binary string.
     Note that 0 is a special case, returning an empty string, to save a
Index: Modules/_pickle.c
===================================================================
--- Modules/_pickle.c	(revision 82908)
+++ Modules/_pickle.c	(working copy)
@@ -3467,28 +3467,16 @@
 instantiate(PyObject *cls, PyObject *args)
 {
     PyObject *r = NULL;
+    Py_ssize_t len = PyObject_Size(args);
+    if (len == -1)
+        return NULL;
+    if (len != 0  || !PyType_Check(cls) ||
+	PyObject_HasAttrString(cls, "__getinitargs__"))
+        r = PyObject_CallObject(cls, args);
+    else
+        r = PyObject_CallMethod(cls, "__new__", "O", cls);
 
-    /* XXX: The pickle.py module does not create instances this way when the
-       args tuple is empty. See Unpickler._instantiate(). */
-    if ((r = PyObject_CallObject(cls, args)))
-        return r;
-
-    /* XXX: Is this still nescessary? */
-    {
-        PyObject *tp, *v, *tb, *tmp_value;
-
-        PyErr_Fetch(&tp, &v, &tb);
-        tmp_value = v;
-        /* NULL occurs when there was a KeyboardInterrupt */
-        if (tmp_value == NULL)
-            tmp_value = Py_None;
-        if ((r = PyTuple_Pack(3, tmp_value, cls, args))) {
-            Py_XDECREF(v);
-            v = r;
-        }
-        PyErr_Restore(tp, v, tb);
-    }
-    return NULL;
+    return r;
 }
 
 static int
@@ -3547,7 +3535,7 @@
         if (len < 2)
             return bad_readline();
         class_name = PyUnicode_DecodeASCII(s, len - 1, "strict");
-        if (class_name == NULL) {
+        if (class_name != NULL) {
             cls = find_class(self, module_name, class_name);
             Py_DECREF(class_name);
         }
@@ -4276,7 +4264,7 @@
         return -1;
     PDATA_POP(self->stack, callable);
     if (callable) {
-        obj = instantiate(callable, argtup);
+        obj = PyObject_CallObject(callable, argtup);
         Py_DECREF(callable);
     }
     Py_DECREF(argtup);