Fix .__class__() calls on builtin types (GH-3956)

Fixes https://github.com/cython/cython/issues/3954

The problem seems to be that __Pyx_CallUnboundCMethod0 optimizes the call incorrectly. This patch avoids it getting sent to that mechanism.

Cython/Compiler/Optimize.py

@@ -3012,6 +3012,13 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
                 is_temp=node.is_temp
             )
 
+    def _handle_any_slot__class__(self, node, function, args,
+                                is_unbound_method, kwargs=None):
+        # The purpose of this function is to handle calls to instance.__class__() so that
+        # it doesn't get handled by the __Pyx_CallUnboundCMethod0 mechanism.
+        # TODO: optimizations of the instance.__class__() call might be possible in future.
+        return node
+
     ### methods of builtin types
 
     PyObject_Append_func_type = PyrexTypes.CFuncType(

Cython/Compiler/Visitor.py

@@ -674,7 +674,7 @@ class MethodDispatcherTransform(EnvTransform):
             "method_%s_%s" % (type_name, attr_name), kwargs)
         if method_handler is None:
             if (attr_name in TypeSlots.method_name_to_slot
-                    or attr_name == '__new__'):
+                    or attr_name in ['__new__', '__class__']):
                 method_handler = self._find_handler(
                     "slot%s" % attr_name, kwargs)
             if method_handler is None:

tests/run/builtin_types_class.py

+# mode: run
+
+from __future__ import print_function
+
+import cython
+
+# https://github.com/cython/cython/issues/3954
+# calls to the __class__ attributes of builtin types were optimized to something invalid
+
+@cython.locals(d=dict)
+def test_dict(d):
+    """
+    >>> test_dict({})
+    dict
+    {}
+    """
+    print(d.__class__.__name__)
+    print(d.__class__())
+
+@cython.locals(i=int)
+def test_int(i):
+    """
+    >>> test_int(0)
+    int
+    0
+    """
+    print(i.__class__.__name__)
+    print(i.__class__())
+
+@cython.cclass
+class C:
+    def __str__(self):
+        return "I'm a C object"
+
+@cython.locals(c=C)
+def test_cdef_class(c):
+    """
+    # This wasn't actually broken but is worth testing anyway
+    >>> test_cdef_class(C())
+    C
+    I'm a C object
+    """
+    print(c.__class__.__name__)
+    print(c.__class__())
+
+@cython.locals(d=object)
+def test_object(o):
+    """
+    >>> test_object({})
+    dict
+    {}
+    >>> test_object(1)
+    int
+    0
+    >>> test_object(C())
+    C
+    I'm a C object
+    """
+    print(o.__class__.__name__)
+    print(o.__class__())