Cannot use Literal variable inside Literal type

[intgr]

Using mypy 0.761, Python 3.7.5 here. The ANIMALS2 value is highlighted as incorrect by Mypy:

COW: Literal['cow'] = 'cow'
PIG: Literal['pig'] = 'pig'
ANIMALS1: Literal['cow', 'pig']  # This is OK
ANIMALS2: Literal[COW, PIG]
#         ^^^^^^^ Mypy: Parameter 1 of Literal[...] is invalid

But it seems Mypy should be able to deduce that ANIMALS1 and ANIMALS2 are the same thing?

I'm not sure if the more lax version should be allowed as well, but seems useful:

OX = 'ox'
ANIMALS = Literal[OX]

[TH3CHARLie]

Well, I find #3494 maybe related to this

[ilevkivskyi]

Well, I find #3494 maybe related to this

Essentially yes, variables and type aliases should be never mixed. If you want an alias, use COW = Literal['cow'].

[TH3CHARLie]

@ilevkivskyi indeed and I misunderstand this, too. However, I found a TODO:

mypy/mypy/typeanal.py

Lines 425 to 427 in fe8309a

	if isinstance(sym.node, Var):
	# TODO: add a link to alias docs, see #3494.
	message = 'Variable "{}" is not valid as a type'

which I think it would be useful to add a note here to point to the docs to help the anyone encounters this error message in the future

Furthermore, I check the docs in both common issues and solutions and alias part and find no specific docs describing this behavior, maybe we should document this and add note in message

[ilevkivskyi]

Yes, but fixing this TODO depends on fixing #3494 first.