allow literal types without Literal

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Basedmypy Changelog
 
 ## [Unreleased]
+### Added
+- Allow literal `int`, `bool` and `Enum`s without `Literal`
 ### Enhancements
 - Unionize at type joins instead of common ancestor
 - Render Literal types better in output messages

mypy/exprtotype.py

@@ -1,5 +1,4 @@
 """Translate an Expression to a Type value."""
-
 from typing import Optional
 
 from mypy.fastparse import parse_type_string
@@ -73,15 +72,27 @@ def expr_to_unanalyzed_type(
     if isinstance(expr, NameExpr):
         name = expr.name
         if name == "True":
-            return RawExpressionType(True, "builtins.bool", line=expr.line, column=expr.column)
+            return RawExpressionType(
+                True,
+                "builtins.bool",
+                line=expr.line,
+                column=expr.column,
+                expression=not allow_new_syntax,
+            )
         elif name == "False":
-            return RawExpressionType(False, "builtins.bool", line=expr.line, column=expr.column)
+            return RawExpressionType(
+                False,
+                "builtins.bool",
+                line=expr.line,
+                column=expr.column,
+                expression=not allow_new_syntax,
+            )
         else:
             return UnboundType(name, line=expr.line, column=expr.column)
     elif isinstance(expr, MemberExpr):
         fullname = get_member_expr_fullname(expr)
         if fullname:
-            return UnboundType(fullname, line=expr.line, column=expr.column)
+            return UnboundType(fullname, line=expr.line, column=expr.column, expression=True)
         else:
             raise TypeTranslationError()
     elif isinstance(expr, IndexExpr):
@@ -193,7 +204,13 @@ def expr_to_unanalyzed_type(
                 return typ
         raise TypeTranslationError()
     elif isinstance(expr, IntExpr):
-        return RawExpressionType(expr.value, "builtins.int", line=expr.line, column=expr.column)
+        return RawExpressionType(
+            expr.value,
+            "builtins.int",
+            line=expr.line,
+            column=expr.column,
+            expression=not allow_new_syntax,
+        )
     elif isinstance(expr, FloatExpr):
         # Floats are not valid parameters for RawExpressionType , so we just
         # pass in 'None' for now. We'll report the appropriate error at a later stage.

mypy/main.py

@@ -608,6 +608,9 @@ def add_invertible_flag(
         help="Partially typed/incomplete functions in this module are considered typed"
         " for untyped call errors.",
     )
+    add_invertible_flag(
+        "--bare-literals", default=True, help="Allow bare literals.", group=based_group
+    )
 
     config_group = parser.add_argument_group(
         title="Config file",

mypy/message_registry.py

@@ -23,6 +23,9 @@ def format(self, *args: object, **kwargs: object) -> "ErrorMessage":
 
 # Invalid types
 INVALID_TYPE_RAW_ENUM_VALUE: Final = "Invalid type: try using Literal[{}.{}] instead?"
+INVALID_BARE_LITERAL: Final = (
+    '"{0}" is a bare literal and cannot be used here, try Literal[{0}] instead?'
+)
 
 # Type checker error message constants
 NO_RETURN_VALUE_EXPECTED: Final = ErrorMessage("No return value expected", codes.RETURN_VALUE)

mypy/options.py

@@ -131,6 +131,7 @@ def __init__(self) -> None:
         self.targets: List[str] = []
         self.ignore_any_from_error = True
         self.incomplete_is_typed = flip_if_not_based(False)
+        self.bare_literals = flip_if_not_based(True)
 
         # disallow_any options
         self.disallow_any_generics = flip_if_not_based(True)

mypy/semanal.py

@@ -3649,8 +3649,15 @@ def process_typevar_parameters(
                     upper_bound = get_proper_type(analyzed)
                     if isinstance(upper_bound, AnyType) and upper_bound.is_from_error:
                         self.fail(message_registry.TYPEVAR_BOUND_MUST_BE_TYPE, param_value)
-                        # Note: we do not return 'None' here -- we want to continue
-                        # using the AnyType as the upper bound.
+                    elif isinstance(upper_bound, LiteralType) and upper_bound.bare_literal:
+                        self.fail(
+                            message_registry.INVALID_BARE_LITERAL.format(upper_bound.value_repr()),
+                            param_value,
+                            code=codes.VALID_TYPE,
+                        )
+
+                    # Note: we do not return 'None' here -- we want to continue
+                    # using the AnyType as the upper bound.
                 except TypeTranslationError:
                     self.fail(message_registry.TYPEVAR_BOUND_MUST_BE_TYPE, param_value)
                     return None

mypy/semanal_newtype.py

@@ -5,7 +5,7 @@
 
 from typing import Optional, Tuple
 
-from mypy import errorcodes as codes
+from mypy import errorcodes as codes, message_registry
 from mypy.errorcodes import ErrorCode
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.messages import MessageBuilder, format_type
@@ -36,6 +36,7 @@
     AnyType,
     CallableType,
     Instance,
+    LiteralType,
     NoneType,
     PlaceholderType,
     TupleType,
@@ -202,10 +203,17 @@ def check_newtype_args(
             should_defer = True
 
         # The caller of this function assumes that if we return a Type, it's always
-        # a valid one. So, we translate AnyTypes created from errors into None.
+        # a valid one. So, we translate AnyTypes created from errors and bare literals into None.
         if isinstance(old_type, AnyType) and old_type.is_from_error:
             self.fail(msg, context)
             return None, False
+        elif isinstance(old_type, LiteralType) and old_type.bare_literal:
+            self.fail(
+                message_registry.INVALID_BARE_LITERAL.format(old_type.value_repr()),
+                context,
+                code=codes.VALID_TYPE,
+            )
+            return None, False
 
         return None if has_failed else old_type, should_defer
 

mypy/typeanal.py

@@ -682,24 +682,32 @@ def analyze_unbound_type_without_type_info(
         # a "Literal[...]" type. So, if `defining_literal` is not set,
         # we bail out early with an error.
         #
-        # If, in the distant future, we decide to permit things like
-        # `def foo(x: Color.RED) -> None: ...`, we can remove that
-        # check entirely.
+        # Based: we permit things like `def foo(x: Color.RED) -> None: ...`
         if isinstance(sym.node, Var) and sym.node.info and sym.node.info.is_enum:
             value = sym.node.name
-            base_enum_short_name = sym.node.info.name
+            # it's invalid to use in an expression, ie: a TypeAlias
             if not defining_literal:
-                msg = message_registry.INVALID_TYPE_RAW_ENUM_VALUE.format(
-                    base_enum_short_name, value
-                )
-                self.fail(msg, t)
-                return AnyType(TypeOfAny.from_error)
-            return LiteralType(
+                if not self.options.bare_literals:
+                    base_enum_short_name = sym.node.info.name
+                    msg = message_registry.INVALID_TYPE_RAW_ENUM_VALUE.format(
+                        base_enum_short_name, value
+                    )
+                    self.fail(msg, t)
+                if t.expression:
+                    base_enum_short_name = sym.node.info.name
+                    name = f"{base_enum_short_name}.{value}"
+                    msg = message_registry.INVALID_BARE_LITERAL.format(name)
+                    self.fail(msg, t)
+            result = LiteralType(
                 value=value,
                 fallback=Instance(sym.node.info, [], line=t.line, column=t.column),
                 line=t.line,
                 column=t.column,
+                bare_literal=True,
             )
+            if t.expression:
+                return result
+            return result.accept(self)
 
         # None of the above options worked. We parse the args (if there are any)
         # to make sure there are no remaining semanal-only types, then give up.
@@ -934,16 +942,18 @@ def visit_raw_expression_type(self, t: RawExpressionType) -> Type:
         # "fake literals" should always be wrapped in an UnboundType
         # corresponding to 'Literal'.
         #
-        # Note: if at some point in the distant future, we decide to
-        # make signatures like "foo(x: 20) -> None" legal, we can change
-        # this method so it generates and returns an actual LiteralType
-        # instead.
+        # Based: signatures like "foo(x: 20) -> None" are legal, this method
+        # generates and returns an actual LiteralType instead.
 
         if self.report_invalid_types:
+            msg = None
             if t.base_type_name in ("builtins.int", "builtins.bool"):
-                # The only time it makes sense to use an int or bool is inside of
-                # a literal type.
-                msg = f"Invalid type: try using Literal[{repr(t.literal_value)}] instead?"
+                if not self.options.bare_literals:
+                    # The only time it makes sense to use an int or bool is inside of
+                    # a literal type.
+                    msg = f"Invalid type: try using Literal[{repr(t.literal_value)}] instead?"
+                if t.expression:
+                    msg = message_registry.INVALID_BARE_LITERAL.format(t.literal_value)
             elif t.base_type_name in ("builtins.float", "builtins.complex"):
                 # We special-case warnings for floats and complex numbers.
                 msg = f"Invalid type: {t.simple_name()} literals cannot be used as a type"
@@ -954,14 +964,38 @@ def visit_raw_expression_type(self, t: RawExpressionType) -> Type:
                 # string, it's unclear if the user meant to construct a literal type
                 # or just misspelled a regular type. So we avoid guessing.
                 msg = "Invalid type comment or annotation"
-
-            self.fail(msg, t, code=codes.VALID_TYPE)
-            if t.note is not None:
-                self.note(t.note, t, code=codes.VALID_TYPE)
-
+            if msg:
+                self.fail(msg, t, code=codes.VALID_TYPE)
+                if t.note is not None:
+                    self.note(t.note, t, code=codes.VALID_TYPE)
+        if t.base_type_name in ("builtins.int", "builtins.bool"):
+            v = t.literal_value
+            assert v is not None
+            result = LiteralType(
+                v,
+                fallback=self.named_type(t.base_type_name),
+                line=t.line,
+                column=t.column,
+                bare_literal=True,
+            )
+            if t.expression:
+                return result
+            return result.accept(self)
         return AnyType(TypeOfAny.from_error, line=t.line, column=t.column)
 
     def visit_literal_type(self, t: LiteralType) -> Type:
+        if (
+            self.nesting_level
+            and t.bare_literal
+            and not (self.api.is_future_flag_set("annotations") or self.api.is_stub_file)
+            and self.options.bare_literals
+        ):
+            self.fail(
+                f'"{t}" is a bare literal and shouldn\'t be used in a type operation without'
+                ' "__future__.annotations"',
+                t,
+                code=codes.VALID_TYPE,
+            )
         return t
 
     def visit_star_type(self, t: StarType) -> Type:

mypy/types.py

@@ -762,6 +762,7 @@ class UnboundType(ProperType):
         "empty_tuple_index",
         "original_str_expr",
         "original_str_fallback",
+        "expression",
     )
 
     def __init__(
@@ -774,6 +775,7 @@ def __init__(
         empty_tuple_index: bool = False,
         original_str_expr: Optional[str] = None,
         original_str_fallback: Optional[str] = None,
+        expression=False,
     ) -> None:
         super().__init__(line, column)
         if not args:
@@ -801,6 +803,9 @@ def __init__(
         self.original_str_expr = original_str_expr
         self.original_str_fallback = original_str_fallback
 
+        self.expression = expression
+        """This is used to ban bare Enum literals from expressions like ``TypeAlias``es"""
+
     def copy_modified(self, args: Bogus[Optional[Sequence[Type]]] = _dummy) -> "UnboundType":
         if args is _dummy:
             args = self.args
@@ -2368,7 +2373,7 @@ class RawExpressionType(ProperType):
         )
     """
 
-    __slots__ = ("literal_value", "base_type_name", "note")
+    __slots__ = ("literal_value", "base_type_name", "note", "expression")
 
     def __init__(
         self,
@@ -2377,11 +2382,13 @@ def __init__(
         line: int = -1,
         column: int = -1,
         note: Optional[str] = None,
+        expression=False,
     ) -> None:
         super().__init__(line, column)
         self.literal_value = literal_value
         self.base_type_name = base_type_name
         self.note = note
+        self.expression = expression
 
     def simple_name(self) -> str:
         return self.base_type_name.replace("builtins.", "")
@@ -2422,15 +2429,21 @@ class LiteralType(ProperType):
     represented as `LiteralType(value="RED", fallback=instance_of_color)'.
     """
 
-    __slots__ = ("value", "fallback", "_hash")
+    __slots__ = ("value", "fallback", "_hash", "bare_literal")
 
     def __init__(
-        self, value: LiteralValue, fallback: Instance, line: int = -1, column: int = -1
+        self,
+        value: LiteralValue,
+        fallback: Instance,
+        line: int = -1,
+        column: int = -1,
+        bare_literal=False,
     ) -> None:
         self.value = value
         super().__init__(line, column)
         self.fallback = fallback
         self._hash = -1  # Cached hash value
+        self.bare_literal = bare_literal
 
     def can_be_false_default(self) -> bool:
         return not self.value

mypy_bootstrap.ini

@@ -14,6 +14,7 @@ warn_unused_configs = True
 show_traceback = True
 always_true = MYPYC
 infer_function_types = True
+bare_literals = True
 
 [mypy-mypy.*]
 default_return = True

mypy_self_check.ini

@@ -30,6 +30,7 @@ no_warn_unreachable = True
 implicit_reexport = True
 disallow_redefinition = True
 disable_error_code = truthy-bool, redundant-expr, ignore-without-code, no-untyped-usage
+bare_literals = True
 
 [mypy-mypy.*,mypyc.*]
 default_return = True