Emit a diagnostic on an attempt to create a "generic enum" #2416
Description
This class statement fails at runtime, but ty does not detect it:
from enum import Enum
class Foo[T](Enum):
X = list[T]()Runtime:
>>> from enum import Enum
...
... class Foo[T](Enum):
... X = list[T]()
...
Traceback (most recent call last):
File "<python-input-0>", line 3, in <module>
class Foo[T](Enum):
X = list[T]()
File "<python-input-0>", line 3, in <generic parameters of Foo>
class Foo[T](Enum):
X = list[T]()
File "/Users/alexw/.pyenv/versions/3.13.1/lib/python3.13/enum.py", line 491, in __prepare__
member_type, first_enum = metacls._get_mixins_(cls, bases)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
File "/Users/alexw/.pyenv/versions/3.13.1/lib/python3.13/enum.py", line 956, in _get_mixins_
raise TypeError("new enumerations should be created as "
"`EnumName([mixin_type, ...] [data_type,] enum_type)`")
TypeError: new enumerations should be created as `EnumName([mixin_type, ...] [data_type,] enum_type)`This also fails in the same way, and again, we emit no error:
from enum import Enum
from typing import Generic, TypeVar
T = TypeVar("T")
class Foo(Enum, Generic[T]):
X = list[T]()The only way you can get a "generic enum" to not immediately fail is by putting Generic[T] first in the bases list (contrary to the usual recommendation that Generic[T] should always be the last base class). But even if you do this, things do not work at runtime in the way you'd expect -- it's illegal to specialize such a class at runtime, because Generic.__class_getitem__ is totally clobbered by EnumMeta.__getitem__:
>> from enum import Enum
>>> from typing import Generic, TypeVar
>>> T = TypeVar("T")
>>> class Foo(Generic[T], Enum):
... X = list[T]()
...
>>> Foo[int]
Traceback (most recent call last):
File "<python-input-5>", line 1, in <module>
Foo[int]
~~~^^^^^
File "/Users/alexw/.pyenv/versions/3.13.1/lib/python3.13/enum.py", line 789, in __getitem__
return cls._member_map_[name]
~~~~~~~~~~~~~~~~^^^^^^
KeyError: <class 'int'>Aside from whether or not it works at runtime, the concept of a generic enum just isn't very coherent on a conceptual level anyway. We currently don't permit them to exist in our internal model, and mypy/pyrefly both emit diagnostics if a user tries to create one:
- https://mypy-play.net/?mypy=latest&python=3.12&gist=35ef91038c460dd48cd557bb8704076d
- https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0BxGOhiUIAYwA0dACrMYANVSUAOujDV6wgK70uPPnQCi6XWrUy6AXlnyllABQqQM5wEpz6cVFRw4dADFcXAchETFxAG0ZAF1pE103RDU6VLp8RFk1EEkQbQZoOBJyRBAAYjoAVQKoCCY6MG0vAtx0OE9MGDAG3hpUBgB9UxpsUQcMznQGNzoAWgA%2BOjgGSmT0NLpKGAZtSnWwZwA5XVHVumB8AF9nbNyyLbAoUkIGWigKCoAFUgenpYwcAQ6OJWpA2Lt%2BhBWoQ1BUAMowGB0AAWDAYxDgiAA9Fj7l0noReGwscIsZhcOI4FiQeoIODKJDWliepQ6KgAG6oaCobCwYGgukQlrrXDEYVFNRkBjI1qzdmiOBQ9Y2ZwAZkIAEYAEw3dAgS65VDiArygLQGAUNBYPBEMj6oA
We should do the same as mypy/pyrefly here.