Prevent C++ coercions from picking up user-set directives (GH-4206)

For example, if they're called on entry/exit to a decorated function they pick up the directives.
They should really be independent of most user defined settings, especially local ones.

Cython/Compiler/PyrexTypes.py

@@ -3808,9 +3808,8 @@ class CppClassType(CType):
                 'type': self.cname,
             })
             # Override directives that should not be inherited from user code.
-            # TODO: filter directives with an allow list to keep only those that are safe and relevant.
-            directives = dict(env.directives, cpp_locals=False)
             from .UtilityCode import CythonUtilityCode
+            directives = CythonUtilityCode.filter_inherited_directives(env.directives)
             env.use_utility_code(CythonUtilityCode.load(
                 cls.replace('unordered_', '') + ".from_py", "CppConvert.pyx",
                 context=context, compiler_directives=directives))
@@ -3855,10 +3854,9 @@ class CppClassType(CType):
                 'maybe_unordered': self.maybe_unordered(),
                 'type': self.cname,
             })
-            # Override directives that should not be inherited from user code.
-            # TODO: filter directives with an allow list to keep only those that are safe and relevant.
-            directives = dict(env.directives, cpp_locals=False)
             from .UtilityCode import CythonUtilityCode
+            # Override directives that should not be inherited from user code.
+            directives = CythonUtilityCode.filter_inherited_directives(env.directives)
             env.use_utility_code(CythonUtilityCode.load(
                 cls.replace('unordered_', '') + ".to_py", "CppConvert.pyx",
                 context=context, compiler_directives=directives))

Cython/Compiler/UtilityCode.py

@@ -227,6 +227,27 @@ class CythonUtilityCode(Code.UtilityCodeBase):
 
         return original_scope
 
+    @staticmethod
+    def filter_inherited_directives(current_directives):
+        """
+        Cython utility code should usually only pick up a few directives from the
+        environment (those that intentionally control its function) and ignore most
+        other compiler directives. This function provides a sensible default list
+        of directives to copy.
+        """
+        from .Options import _directive_defaults
+        utility_code_directives = dict(_directive_defaults)
+        inherited_directive_names = (
+            'binding', 'always_allow_keywords', 'allow_none_for_extension_args',
+            'auto_pickle', 'ccomplex',
+            'c_string_type', 'c_string_encoding',
+            'optimize.inline_defnode_calls', 'optimize.unpack_method_calls',
+            'optimize.unpack_method_calls_in_pyinit', 'optimize.use_switch')
+        for name in inherited_directive_names:
+            if name in current_directives:
+                utility_code_directives[name] = current_directives[name]
+        return utility_code_directives
+
 
 def declare_declarations_in_scope(declaration_string, env, private_type=True,
                                   *args, **kwargs):