Index: Python/ast.c
===================================================================
--- Python/ast.c	(revision 54277)
+++ Python/ast.c	(working copy)
@@ -1530,7 +1530,7 @@
        constant.  The peephole optimizer already does something like
        this but it doesn't handle the case where the constant is
        (sys.maxint - 1).  In that case, we want a PyIntObject, not a
-       PyLongObject.
+       PyLongObject.  However, preserve the sign of -0.0.
     */
     if (TYPE(CHILD(n, 0)) == MINUS
 	&& NCH(n) == 2
@@ -1540,14 +1540,22 @@
 	&& NCH(ppower) == 1
 	&& TYPE((patom = CHILD(ppower, 0))) == atom
 	&& TYPE((pnum = CHILD(patom, 0))) == NUMBER) {
-	char *s = PyObject_MALLOC(strlen(STR(pnum)) + 2);
-	if (s == NULL)
-	    return NULL;
-	s[0] = '-';
-	strcpy(s + 1, STR(pnum));
-	PyObject_FREE(STR(pnum));
-	STR(pnum) = s;
-	return ast_for_atom(c, patom);
+        PyObject* thenum = parsenumber(STR(pnum));
+        if (thenum == NULL)
+            return NULL;
+        if (!PyFloat_Check(thenum) || PyObject_IsTrue(thenum)) {
+	    char *s = PyObject_MALLOC(strlen(STR(pnum)) + 2);
+            if (s == NULL)
+                return NULL;
+            s[0] = '-';
+            strcpy(s + 1, STR(pnum));
+            PyObject_FREE(STR(pnum));
+            STR(pnum) = s;
+            Py_DECREF(thenum);
+            return ast_for_atom(c, patom);
+        } else {
+            Py_DECREF(thenum);
+        }
     }
 
     expression = ast_for_expr(c, CHILD(n, 1));
Index: Lib/test/test_float.py
===================================================================
--- Lib/test/test_float.py	(revision 54277)
+++ Lib/test/test_float.py	(working copy)
@@ -81,6 +81,7 @@
 # on an IEEE platform, all we guarantee is that bit patterns
 # representing infinities or NaNs do not raise an exception; all else
 # is accident (today).
+# let's also try to guarantee that -0.0 and 0.0 don't get confused.
 
 class IEEEFormatTestCase(unittest.TestCase):
     if float.__getformat__("double").startswith("IEEE"):
@@ -99,7 +100,21 @@
                               ('<f', LE_FLOAT_NAN)]:
                 struct.unpack(fmt, data)
 
+    if float.__getformat__("double").startswith("IEEE"):
+        def test_negative_zero(self):
+            import math
+            def pos_pos():
+                return 0.0, math.atan2(0.0, -1)
+            def pos_neg():
+                return 0.0, math.atan2(-0.0, -1)
+            def neg_pos():
+                return -0.0, math.atan2(0.0, -1)
+            def neg_neg():
+                return -0.0, math.atan2(-0.0, -1)
+            self.assertEquals(pos_pos(), neg_pos())
+            self.assertEquals(pos_neg(), neg_neg())
 
+
 def test_main():
     test_support.run_unittest(
         FormatFunctionsTestCase,