Allow returning Literals in __new__

mypy/checker.py

@@ -1475,7 +1475,8 @@ def check___new___signature(self, fdef: FuncDef, typ: CallableType) -> None:
                 "but must return a subtype of",
             )
         elif not isinstance(
-            get_proper_type(bound_type.ret_type), (AnyType, Instance, TupleType, UninhabitedType)
+            get_proper_type(bound_type.ret_type),
+            (AnyType, Instance, TupleType, UninhabitedType, LiteralType),
         ):
             self.fail(
                 message_registry.NON_INSTANCE_NEW_TYPE.format(

test-data/unit/check-classes.test

@@ -473,6 +473,41 @@ class B(A):
     def __new__(cls) -> B:
         pass
 
+[case testOverride__new__WithLiteralReturnPassing]
+[file builtins.py]
+from typing import Literal, Protocol, overload
+
+class str: pass
+class dict: pass
+class float: pass
+
+class _Truthy(Protocol):
+    def __bool__(self) -> Literal[True]: pass
+
+class _Falsy(Protocol):
+    def __bool__(self) -> Literal[False]: pass
+
+class bool(int):
+    @overload
+    def __new__(cls, __o: _Truthy) -> Literal[True]: pass
+    @overload
+    def __new__(cls, __o: _Falsy) -> Literal[False]: pass
+    def __new__(cls, __o: object):
+        pass
+
+class int:
+    def __new__(cls) -> Literal[0]: pass
+[typing fixtures/typing-medium.pyi]
+
+[case testOverride__new__WithLiteralReturnFailing]
+from typing import Literal
+
+class Foo:
+    def __new__(cls) -> Literal[1]: pass # E: Incompatible return type for "__new__" (returns "Literal[1]", but must return a subtype of "Foo")
+
+[builtins fixtures/__new__.pyi]
+[typing fixtures/typing-medium.pyi]
+
 [case testOverride__new__AndCallObject]
 from typing import TypeVar, Generic
 

test-data/unit/fixtures/__new__.pyi

@@ -12,6 +12,7 @@ class object:
 class type:
     def __init__(self, x) -> None: pass
 
+class float: pass
 class int: pass
 class bool: pass
 class str: pass