Add support for TypeAliasType

[ks-mw]

Bug Report

Creating a simple TypeAliasType results in mypy reporting a type error.

To Reproduce

from typing_extensions import TypeAliasType
TestType = TypeAliasType("TestType", int)
x: TestType = 42

Playground:
https://mypy-play.net/?mypy=latest&python=3.11&gist=294b3a573ab591aa3026f7c05645c809

Gist:
https://gist.github.com/mypy-play/294b3a573ab591aa3026f7c05645c809

Expected Behavior

No type error should be reported.

Actual Behavior

mypy reports: main.py:3: error: Variable "__main__.TestType" is not valid as a type [valid-type]

Your Environment

• Mypy version used: 1.7.1
• Python version used: 3.11, 3.10

[JelleZijlstra]

TypeAliasType is not supported yet. See also #15238. PRs welcome!

[ks-mw]

Thank you for the information!

[sobolevn]

Do we really want to support this? I thought that TypeAliasType is an internal thing needed to support dynamic checks for constructs like type A = ... and isinstance(A, TypeAliasType).

I don't think that we need to promote the usage OP is proposing.

[hmc-cs-mdrissi]

It’s useful same way using typing extensions helps. If you are on 3.11 or lower then you can’t use type syntax, but it would be nice to use new type aliases especially as they have explicit type variables and there are some things that are only possible with new syntax.

type First[S, T] = S

is an alias that is possible to write using TypeAliasType but impossible in 3.11 or earlier without it. I thought PEP exposing this as public and documented thing was to allow for this. Additionally typing extensions has it too which it wouldn’t need if intended to be internal.

Lastly pyright does support this.

[JelleZijlstra]

Yes, we discussed this on typing-sig and it's intended for this usage to be supported on Python 3.11 and lower.

[tmke8]

#16926 added support for this, but I noticed that it's not supported within classes:

from typing_extensions import TypeAliasType

class A:
    T1 = TypeAliasType("T1", int)
    
x: A.T1 = 2

playground: https://mypy-play.net/?mypy=master&python=3.12&gist=f0b984556ec1fa1fa5cdef645bf3561f

I'm not 100% sure this should be supported, but it does work on pyright.