Improve the error message for "Optional[ctype]". Previously, it failed...

Improve the error message for "Optional[ctype]". Previously, it failed complaining about "or None", which wasn't the actual syntax construct on user side.

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

Cython/Compiler/Nodes.py

@@ -968,8 +968,11 @@ class CArgDeclNode(Node):
 
         if arg_type and arg_type.python_type_constructor_name == "typing.Optional":
             # "x: Optional[...]"  =>  explicitly allow 'None'
-            self.or_none = True
             arg_type = arg_type.resolve()
+            if arg_type and not arg_type.is_pyobject:
+                error(annotation.pos, "Only Python type arguments can use typing.Optional[...]")
+            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'
             self.or_none = True

tests/errors/e_typing_optional.py

+# mode: error
+
+import cython
+
+try:
+    from typing import Optional
+except ImportError:
+    pass
+
+
+def optional_pytypes(i: Optional[int], f: Optional[float]):
+    pass
+
+
+def optional_cython_types(i: Optional[cython.int], d: Optional[cython.double], f: Optional[cython.float]):
+    pass
+
+
+MyStruct = cython.struct(a=cython.int, b=cython.double)
+
+def optional_cstruct(x: Optional[MyStruct]):
+    pass
+
+
+_ERRORS = """
+15:29: Only Python type arguments can use typing.Optional[...]
+15:54: Only Python type arguments can use typing.Optional[...]
+15:82: Only Python type arguments can use typing.Optional[...]
+21:24: Only Python type arguments can use typing.Optional[...]
+
+# FIXME: these should be allowed!
+11:24: Only Python type arguments can use typing.Optional[...]
+11:42: Only Python type arguments can use typing.Optional[...]
+"""