More bug 460020: when F is a subclass of float, disable the unary plus

optimization (+F(whatever)).

Lib/test/test_descr.py

@@ -1401,6 +1401,7 @@ def inherits():
     a = precfloat(12345)
     verify(float(a) == 12345.0)
     verify(float(a).__class__ is float)
+    verify((+a).__class__ is float)
 
     class madtuple(tuple):
         _rev = None

Objects/floatobject.c

@@ -553,8 +553,12 @@ float_neg(PyFloatObject *v)
 static PyObject *
 float_pos(PyFloatObject *v)
 {
-	Py_INCREF(v);
-	return (PyObject *)v;
+	if (PyFloat_CheckExact(v)) {
+		Py_INCREF(v);
+		return (PyObject *)v;
+	}
+	else
+		return PyFloat_FromDouble(v->ob_fval);
 }
 
 static PyObject *