Restore error when using arbitrary decorators on cdef functions (GH-4323)

These were lost when cdef properties (for extern types) were introduced.

Closes #4322 (based on my interpretation of the problem as an error-reporting issue)

Cython/Compiler/ParseTreeTransforms.py

@@ -1123,6 +1123,7 @@ class InterpretCompilerDirectives(CythonTransform):
                 realdecs.append(dec)
         if realdecs and (scope_name == 'cclass' or
                          isinstance(node, (Nodes.CClassDefNode, Nodes.CVarDefNode))):
+            # Note - arbitrary C function decorators are caught later in DecoratorTransform
             raise PostParseError(realdecs[0].pos, "Cdef functions/classes cannot take arbitrary decorators.")
         node.decorators = realdecs[::-1] + both[::-1]
         # merge or override repeated directives
@@ -1438,7 +1439,14 @@ class DecoratorTransform(ScopeTrackingTransform, SkipDeclarations):
 
     def visit_CFuncDefNode(self, node):
         node = self.visit_FuncDefNode(node)
-        if self.scope_type != 'cclass' or self.scope_node.visibility != "extern" or not node.decorators:
+        if not node.decorators:
+            return node
+        elif self.scope_type != 'cclass' or self.scope_node.visibility != "extern":
+            # at the moment cdef functions are very restricted in what decorators they can take
+            # so it's simple to test for the small number of allowed decorators....
+            if not (len(node.decorators) == 1 and node.decorators[0].decorator.is_name and
+                    node.decorators[0].decorator.name == "staticmethod"):
+                error(node.decorators[0].pos, "Cdef functions cannot take arbitrary decorators.")
             return node
 
         ret_node = node

tests/errors/cdef_func_decorators.pyx

+# mode: error
+# tag: decorator
+
+from functools import wraps
+
+@wraps
+cdef cant_be_decoratored():
+    pass
+
+@wraps
+cpdef also_cant_be_decorated():
+    pass
+
+cdef class C:
+    @wraps
+    cdef still_cant_be_decorated(self):
+        pass
+
+    @property
+    cdef property_only_works_for_extern_classes(self):
+        pass
+
+    @wraps
+    cpdef also_still_cant_be_decorated(self):
+        pass
+
+    @wraps
+    @wraps
+    cdef two_is_just_as_bad_as_one(self):
+        pass
+
+_ERRORS = """
+6:0: Cdef functions cannot take arbitrary decorators.
+10:0: Cdef functions cannot take arbitrary decorators.
+15:4: Cdef functions cannot take arbitrary decorators.
+19:4: Cdef functions cannot take arbitrary decorators.
+23:4: Cdef functions cannot take arbitrary decorators.
+27:4: Cdef functions cannot take arbitrary decorators.
+"""