fix weird edge case for is_none_type with python3.8

pydantic/typing.py

@@ -275,7 +275,7 @@ NoneType = None.__class__
 NONE_TYPES: Tuple[Any, Any, Any] = (None, NoneType, Literal[None])
 
 
-if sys.version_info < (3, 8):  # noqa: C901 (ignore complexity)
+if sys.version_info < (3, 8):
     # Even though this implementation is slower, we need it for python 3.6/3.7:
     # In python 3.6/3.7 "Literal" is not a builtin type and uses a different
     # mechanism.
@@ -285,6 +285,18 @@ if sys.version_info < (3, 8):  # noqa: C901 (ignore complexity)
     def is_none_type(type_: Any) -> bool:
         return type_ in NONE_TYPES
 
+elif sys.version_info[:2] == (3, 8):
+    # We can use the fast implementation for 3.8 but there is a very weird bug
+    # where it can fail for `Literal[None]`.
+    # We just need to redefine a useless `Literal[None]` inside the function body to fix this
+
+    def is_none_type(type_: Any) -> bool:
+        Literal[None]  # fix edge case
+        for none_type in NONE_TYPES:
+            if type_ is none_type:
+                return True
+        return False
+
 else:
 
     def is_none_type(type_: Any) -> bool: