Follow PEP-484 and warn about "func(x: list = None)" without an explicit...

Follow PEP-484 and warn about "func(x: list = None)" without an explicit "typing.Optional[]" declaration.

See https://github.com/cython/cython/issues/3883

Cython/Compiler/Nodes.py

@@ -974,7 +974,9 @@ class CArgDeclNode(Node):
             else:
                 self.or_none = True
         elif arg_type and arg_type.is_pyobject and self.default and self.default.is_none:
-            # "x: ... = None"  =>  implicitly allow 'None'
+            # "x: ... = None"  =>  implicitly allow 'None', but warn about it.
+            if not self.or_none:
+                warning(self.pos, "PEP-484 recommends 'typing.Optional[...]' for arguments that can be None.")
                 self.or_none = True
         elif arg_type and arg_type.is_pyobject and not self.or_none:
             self.not_none = True

tests/run/annotation_typing.pyx

@@ -332,6 +332,9 @@ _WARNINGS = """
 14:77: Dicts should no longer be used as type annotations. Use 'cython.int' etc. directly.
 14:85: Python type declaration in signature annotation does not refer to a Python type
 14:85: Strings should no longer be used for type declarations. Use 'cython.int' etc. directly.
+36:64: PEP-484 recommends 'typing.Optional[...]' for arguments that can be None.
+63:68: PEP-484 recommends 'typing.Optional[...]' for arguments that can be None.
+90:68: PEP-484 recommends 'typing.Optional[...]' for arguments that can be None.
 274:44: Unknown type declaration in annotation, ignoring
 281:29: Ambiguous types in annotation, ignoring
 298:15: Annotation ignored since class-level attributes must be Python objects. Were you trying to set up an instance attribute?