typing_extensions.NewType annotations are not recognized on < 3.10 (regression in 1.4.0) #16297

Bug Report

Mypy 1.4.0 introduced a regression where typing_extensions.NewType annotations are not correctly recognized when --python-version < 3.10.

this bisects to python/typeshed@a6d0aaf via python/typeshed#10205 via #15334. however this does not appear to be a typeshed issue; it's just that the typeshed change exposed a previously-hidden Mypy issue.

To Reproduce

# run with --python-version=3.9 or below, using mypy d11db28 or 1.4 or later

from typing_extensions import NewType

N = NewType("N", int)

x: N  # error: Variable "eg_newtype.N" is not valid as a type  [valid-type]

Expected Behavior

Mypy passes this example.

Actual Behavior

Mypy fails this example; see above.

Your Environment

• Mypy version used: 1.6.1 (compiled) and current master (uncompiled)
• Mypy command-line flags: --python-version=3.9
• Mypy configuration options from mypy.ini (and other config files): none (i.e. using vendored typeshed)
• Python version used: 3.12