Enforce consistent use of Literal and None

pyi.py

@@ -1359,7 +1359,7 @@ def visit_Subscript(self, node: ast.Subscript) -> None:
         self.visit(subscripted_object)
         if subscripted_object_name == "Literal":
             with self.string_literals_allowed.enabled():
-                self.visit(node.slice)
+                self._visit_typing_Literal(node)
             return
 
         if isinstance(node.slice, ast.Tuple):
@@ -1369,6 +1369,33 @@ def visit_Subscript(self, node: ast.Subscript) -> None:
             if subscripted_object_name in {"tuple", "Tuple"}:
                 self._Y090_error(node)
 
+    def _visit_typing_Literal(self, node: ast.Subscript) -> None:
+        if isinstance(node.slice, ast.Constant) and _is_None(node.slice):
+            # Special case for `Literal[None]`
+            self.error(node.slice, Y061.format(suggestion="None"))
+        elif isinstance(node.slice, ast.Tuple):
+            elts = node.slice.elts
+            for i, elt in enumerate(elts):
+                if _is_None(elt):
+                    elts_without_none = elts[:i] + elts[i + 1 :]
+                    slice_without_none = (
+                        elts_without_none[0]
+                        if len(elts_without_none) == 1
+                        else ast.Tuple(elts=elts_without_none)
+                    )
+                    suggestion = unparse(
+                        ast.BinOp(
+                            op=ast.BitOr(),
+                            left=ast.Subscript(
+                                value=node.value, slice=slice_without_none
+                            ),
+                            right=ast.Constant(value=None),
+                        )
+                    )
+                    self.error(elt, Y061.format(suggestion=suggestion))
+                    break  # Only report the first `None`
+        self.visit(node.slice)
+
     def _visit_slice_tuple(self, node: ast.Tuple, parent: str | None) -> None:
         if parent == "Union":
             self._check_union_members(node.elts, is_pep_604_union=False)
@@ -2180,6 +2207,7 @@ def parse_options(options: argparse.Namespace) -> None:
     'Y060 Redundant inheritance from "Generic[]"; '
     "class would be inferred as generic anyway"
 )
+Y061 = "Y061 None inside Literal[...] expression. Replace with {suggestion}"
 Y090 = (
     'Y090 "{original}" means '
     '"a tuple of length 1, in which the sole element is of type {typ!r}". '

tests/literals.pyi

@@ -0,0 +1,6 @@
+from typing import Literal
+
+Literal[None]  # Y061 None inside Literal[...] expression. Replace with None
+Literal[True, None]  # Y061 None inside Literal[...] expression. Replace with Literal[True] | None
+Literal[1, None, "foo"]  # Y061 None inside Literal[...] expression. Replace with Literal[1, 'foo'] | None
+Literal[None, None]  # Y061 None inside Literal[...] expression. Replace with Literal[None] | None