Change IndexError to TypeError when calling unbound CyFunctions (GH-4783)

Cython/Utility/CythonFunction.c

@@ -660,6 +660,9 @@ static PyObject *__Pyx_CyFunction_CallAsMethod(PyObject *func, PyObject *args, P
         self = PyTuple_GetItem(args, 0);
         if (unlikely(!self)) {
             Py_DECREF(new_args);
+            PyErr_Format(PyExc_TypeError,
+                         "unbound method %.200S() needs an argument",
+                         cyfunc->func_qualname);
             return NULL;
         }
 

tests/run/cyfunction.pyx

@@ -363,6 +363,9 @@ cdef class TestUnboundMethodCdef:
     >>> C = TestUnboundMethodCdef
     >>> IS_PY2 or (C.meth is C.__dict__["meth"])
     True
+    >>> TestUnboundMethodCdef.meth()  # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: ...
     """
     def meth(self): pass
 
@@ -372,6 +375,9 @@ class TestUnboundMethod:
     >>> C = TestUnboundMethod
     >>> IS_PY2 or (C.meth is C.__dict__["meth"])
     True
+    >>> TestUnboundMethod.meth()  # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: ...
     """
     def meth(self): pass