Variable "typing_extensions.ReadOnly" is not valid as a type

[MarcoGorelli]

Bug Report

ReadOnly is in typing-extensions but we can't use it with MyPy (but we can with PyRight)

To Reproduce

from typing import TypedDict, Hashable
from typing_extensions import ReadOnly

class Animal(TypedDict):
    name: ReadOnly[Hashable]
    
class Cat(TypedDict):
    name: str

cat: Animal(name='Toby')

https://gist.github.com/mypy-play/39923f5d02befb8d559583b702103391

Expected Behavior

No issues reported (which is what happens with PyRight)

Actual Behavior

main.py:5: error: Variable "typing_extensions.ReadOnly" is not valid as a type  [valid-type]
main.py:5: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
main.py:10: error: Invalid type comment or annotation  [valid-type]
main.py:10: note: Suggestion: use Animal[...] instead of Animal(...)
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.11.12
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[brianschubert]

PEP 705 / ReadOnly support is currently being tracked in #17264, and should land with #17644 🎉