Error when only assigning one of two typevars to concrete types (but no error when assigning two or zero)

Bug Report

When writing a generic class with two typevars, mypy accepts classmethods that (1) assign values based on both typevars and (2) assigns values based on no typevars; but does not accept a classmethod that assigns one value based on a typevar and one value based on a concrete value.

https://stackoverflow.com/q/74417503/1669576

To Reproduce

from typing import Generic, TypeVar

U = TypeVar('U')
V = TypeVar('V')
class Example(Generic[U, V]):
    def __init__(self, a: U, b: V):
        self._a = a
        self._b = b

    @classmethod
    def first(cls, val: U):
        return cls(val, None)

Expected Behavior

mypy should infer that None binds to V

Actual Behavior

mypy gives a seemingly spurious error error: Argument 2 to "Example" has incompatible type "None"; expected "V" [arg-type]

Your Environment

name: test
channels:
  - conda-forge
  - defaults
dependencies:
  - _libgcc_mutex=0.1=conda_forge
  - _openmp_mutex=4.5=2_kmp_llvm
  - bzip2=1.0.8=h7b6447c_0
  - ca-certificates=2022.9.24=ha878542_0
  - certifi=2022.9.24=pyhd8ed1ab_0
  - ld_impl_linux-64=2.38=h1181459_1
  - libffi=3.3=he6710b0_2
  - libgcc-ng=12.2.0=h65d4601_19
  - libstdcxx-ng=11.2.0=h1234567_1
  - libuuid=1.41.5=h5eee18b_0
  - llvm-openmp=14.0.6=h9e868ea_0
  - mypy=0.990=py310h5764c6d_2
  - mypy_extensions=0.4.3=py310h06a4308_0
  - ncurses=6.3=h5eee18b_3
  - openssl=1.1.1s=h166bdaf_0
  - pip=22.2.2=py310h06a4308_0
  - psutil=5.9.0=py310h5eee18b_0
  - python=3.10.6=haa1d7c7_1
  - python_abi=3.10=2_cp310
  - readline=8.2=h5eee18b_0
  - setuptools=65.5.0=py310h06a4308_0
  - sqlite=3.39.3=h5082296_0
  - tk=8.6.12=h1ccaba5_0
  - toml=0.10.2=pyhd3eb1b0_0
  - tomli=2.0.1=pyhd8ed1ab_0
  - typing_extensions=4.3.0=py310h06a4308_0
  - tzdata=2022f=h04d1e81_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.2.6=h5eee18b_0
  - zlib=1.2.13=h5eee18b_0
prefix: /home/b/miniconda3/envs/test

• Mypy version used: 0.990
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.10.6

Inizia eseguendo la classe Example fornita con mypy 0.990 e senza flag di configurazione per riprodurre l'errore. Traccia quindi l'inferenza dei metodi di classe generici e delle variabili di tipo nel type checker, poi aggiungi un test di regressione che copra un argomento concreto e uno di variabile di tipo; il lavoro è completo quando mypy accetta l'esempio senza indebolire gli errori esistenti per zero o due variabili di tipo.