Disallow <double*>obj (#313). Also removes warning for <void*>obj.

Cython/Compiler/ExprNodes.py

@@ -3937,6 +3937,8 @@ class TypecastNode(NewTempExprNode):
         elif from_py and not to_py:
             if self.type.from_py_function:
                 self.operand = self.operand.coerce_to(self.type, env)
+            elif self.type.is_ptr and not self.type.base_type.is_void:
+                error(self.pos, "Python objects can only be cast to void*")
             else:
                 warning(self.pos, "No conversion from %s to %s, python object pointer used." % (self.type, self.operand.type))
         elif from_py and to_py:

tests/errors/pyobjcastdisallow_T313.pyx

+
+a = 3
+
+cdef void* allowed = <void*>a
+cdef double* disallowed = <double*>a
+
+_ERRORS = u"""
+5:26: Python objects can only be cast to void*
+"""

tests/run/pyobjcast_T313.pyx

+# Ensure casting still works to void*
+
+"""
+>>> f()
+('teststring', 'teststring')
+"""
+
+cdef extern from *:
+    ctypedef void PyObject
+
+def f():
+    cdef void* p1
+    cdef PyObject* p2
+    a = "teststring"
+    p1 = <void*>a
+    p2 = <PyObject*>a
+    return (<object>p1, <object>p2)
+