Reject direct union expressions in Literal

Author: 97littleleaf11

mypy/typeanal.py

@@ -1730,6 +1730,14 @@ def analyze_literal_param(self, idx: int, arg: Type, ctx: Context) -> list[Type]
                 )
             ]
 
+        if self.is_direct_union_literal_param(arg):
+            self.fail(
+                f"Parameter {idx} of Literal[...] cannot be a union expression",
+                ctx,
+                code=codes.VALID_TYPE,
+            )
+            return None
+
         # If arg is an UnboundType that was *not* originally defined as
         # a string, try expanding it in case it's a type alias or something.
         if isinstance(arg, UnboundType):
@@ -1786,6 +1794,9 @@ def analyze_literal_param(self, idx: int, arg: Type, ctx: Context) -> list[Type]
             # Types generated from declarations like "var: Final = 4".
             return [arg.last_known_value]
         elif isinstance(arg, UnionType):
+            # This handles unions produced by expanding aliases or nested Literal types,
+            # such as Literal[Alias] where Alias = Literal[1, None]. Direct union
+            # expressions inside Literal[...] are rejected before alias expansion above.
             out = []
             for union_arg in arg.items:
                 union_result = self.analyze_literal_param(idx, union_arg, ctx)
@@ -1797,6 +1808,21 @@ def analyze_literal_param(self, idx: int, arg: Type, ctx: Context) -> list[Type]
             self.fail(f"Parameter {idx} of Literal[...] is invalid", ctx, code=codes.VALID_TYPE)
             return None
 
+    def is_direct_union_literal_param(self, arg: Type) -> bool:
+        """Is this a direct Union expression inside Literal[...]?"""
+        if (
+            isinstance(arg, ProperType)
+            and isinstance(arg, UnionType)
+            and arg.uses_pep604_syntax
+            and arg.original_str_expr is None
+        ):
+            return True
+        if isinstance(arg, UnboundType) and arg.args:
+            sym = self.lookup_qualified(arg.name, arg, suppress_errors=True)
+            if sym is not None and sym.node is not None:
+                return sym.node.fullname in ("typing.Union", "typing.Optional")
+        return False
+
     def analyze_type(self, typ: Type) -> Type:
         return typ.accept(self)
 

test-data/unit/check-literal.test

@@ -611,6 +611,21 @@ e: Literal[dummy()]             # E: Invalid type: Literal[...] cannot contain a
 [builtins fixtures/tuple.pyi]
 [out]
 
+[case testLiteralDisallowUnionExpressions]
+from typing import Literal, Optional, Union
+from typing import Literal as L, Optional as O, Union as U
+
+a: Literal[1 | None]                 # E: Parameter 1 of Literal[...] cannot be a union expression
+b: Literal[1 | 2]                    # E: Parameter 1 of Literal[...] cannot be a union expression
+c: Literal[Literal[1] | None]        # E: Parameter 1 of Literal[...] cannot be a union expression
+d: Literal[Union[Literal[1], None]]  # E: Parameter 1 of Literal[...] cannot be a union expression
+e: Literal[Optional[Literal[1]]]     # E: Parameter 1 of Literal[...] cannot be a union expression
+f: L[1 | None]                       # E: Parameter 1 of Literal[...] cannot be a union expression
+g: Literal[U[Literal[1], None]]      # E: Parameter 1 of Literal[...] cannot be a union expression
+h: Literal[O[Literal[1]]]            # E: Parameter 1 of Literal[...] cannot be a union expression
+[builtins fixtures/tuple.pyi]
+[out]
+
 [case testLiteralDisallowCollections]
 from typing import Literal
 a: Literal[{"a": 1, "b": 2}]    # E: Parameter 1 of Literal[...] is invalid
@@ -688,6 +703,32 @@ reveal_type(e)   # N: Revealed type is "None | None | None"
 [builtins fixtures/bool.pyi]
 [out]
 
+[case testLiteralValidNoneUnionAlias]
+from typing import Literal, Union
+
+a: Literal[1, None]
+b: Literal[Literal[1, None]]
+
+Alias1 = Literal[1, None]
+c: Literal[Alias1]
+
+Alias2 = Literal[1] | None
+d: Literal[Alias2]
+
+Alias3 = Union[Literal[1], None]
+e: Literal[Alias3]
+
+f: Literal["1 | None"]
+
+reveal_type(a)  # N: Revealed type is "Literal[1] | None"
+reveal_type(b)  # N: Revealed type is "Literal[1] | None"
+reveal_type(c)  # N: Revealed type is "Literal[1] | None"
+reveal_type(d)  # N: Revealed type is "Literal[1] | None"
+reveal_type(e)  # N: Revealed type is "Literal[1] | None"
+reveal_type(f)  # N: Revealed type is "Literal['1 | None']"
+[builtins fixtures/tuple.pyi]
+[out]
+
 [case testLiteralMultipleValuesExplicitTuple]
 from typing import Literal
 # Unfortunately, it seems like typed_ast is unable to distinguish this from