TYPE_CHECKING-only `Final`-annotated nested `always-true/false` flags crashes with conditional type aliases

[bzoracler]

Crash Report & To Reproduce

See mypy Playground (reproduces on both 1.15 and 1.16).

The motivation in this snippet is to avoid runtime imports from typing.

# mypy: always-false=INTERNAL_DEVELOPMENT_FLAG

TYPE_CHECKING: Final = False
INTERNAL_DEVELOPMENT_FLAG: Final = False
if TYPE_CHECKING:
 from typing_extensions import TypeAliasType, Final, TypeVar

 T1 = TypeVar("T1")
 T2 = TypeVar("T2", default=T1)
 if INTERNAL_DEVELOPMENT_FLAG:
 Alias = TypeAliasType("Alias", "MyClass", type_params=(T1, T2))
 else:
 Alias = TypeAliasType("Alias", T1, type_params=(T1, T2))

class MyClass(tuple["T1", "T2"]): ...

The crash seems to be avoidable if the flags aren't annotated with Final, or the typing.* imports aren't under one of the flags.

Traceback

Deferral trace:
 __main__:3
 __main__:3
 __main__:3
 __main__:3
 __main__:3
 __main__:4
 __main__:4
 __main__:4
 __main__:4
 __main__:4
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
 __main__:13
main.py: error: INTERNAL ERROR: maximum semantic analysis iteration count reached
Found 1 error in 1 file (errors prevented further checking)

Your Environment

• Mypy version used: 1.15, 1.16
• Mypy command-line flags: --show-traceback
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9, 3.12