Treat Any metaclass the same as Any base class

[sobolevn]

Closes#13599

[sobolevn]

Also disallow_subclassing_any need to be covered 🤔

[AlexWaygood]

Maybe add a couple of tests to check/clarify that this only affects class objects, not instances of classes?

Suggested change

	reveal_type(WithMeta.x) # N: Revealed type is "builtins.int"
	reveal_type(WithMeta.x) # N: Revealed type is "builtins.int"
	reveal_type(WithMeta().a) # E: "WithMeta" has no attribute "a" \
	# N: Revealed type is "Any"
	reveal_type(WithMeta().x) # N: Revealed type is "builtins.int"

[sobolevn]

Sure!

[AlexWaygood]

Can't you just do this?

[AlexWaygood]

I don't feel qualified to comment on whether this is the best way to implement this, but the tests look great!

[sobolevn]

@AlexWaygood you can see that I was copying the approach we have for base classes:

mypy/mypy/semanal.py

Lines 1894 to 1900 in 9d21615

	ifself.options.disallow_subclassing_any:
	ifisinstance(base_expr, (NameExpr, MemberExpr)):
	msg=f'Class cannot subclass "{base_expr.name}" (has type "Any")'
	else:
	msg='Class cannot subclass value of type "Any"'
	self.fail(msg, base_expr)
	info.fallback_to_any=True

Basically, it uses this mechanism:

mypy/mypy/nodes.py

Lines 2738 to 2743 in 9d21615

	# If true, any unknown attributes should have type 'Any' instead
	# of generating a type error. This would be true if there is a
	# base class with type 'Any', but other use cases may be
	# possible. This is similar to having __getattr__ that returns Any
	# (and __setattr__), but without the __getattr__ method.
	fallback_to_any: bool

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[hauntsaninja]

Looks great!