Add 'cpow' as a forward compatibility to 0.29.x (GH-5060)

See #5016

Cython/Compiler/ExprNodes.py

@@ -11779,6 +11779,14 @@ def py_operation_function(self, code):
 class PowNode(NumBinopNode):
     #  '**' operator.
 
+    def analyse_types(self, env):
+        if not env.directives['cpow']:
+            # Note - the check here won't catch cpow directives that don't use '**'
+            # but that's probably OK for a placeholder forward compatibility directive
+            error(self.pos, "The 'cpow' directive is provided for forward compatibility "
+                  "and must be True")
+        return super(PowNode, self).analyse_types(env)
+
     def analyse_c_operation(self, env):
         NumBinopNode.analyse_c_operation(self, env)
         if self.type.is_complex:

Cython/Compiler/Options.py

@@ -179,6 +179,7 @@ def get_directive_defaults():
     'cdivision': False,  # was True before 0.12
     'cdivision_warnings': False,
     'c_api_binop_methods': True,
+    'cpow': True,
     'overflowcheck': False,
     'overflowcheck.fold': True,
     'always_allow_keywords': False,
@@ -318,6 +319,7 @@ def normalise_encoding_name(option_name, encoding):
     'freelist': int,
     'c_string_type': one_of('bytes', 'bytearray', 'str', 'unicode'),
     'c_string_encoding': normalise_encoding_name,
+    'cpow': bool
 }
 
 for key, val in _directive_defaults.items():