TypeForm does not overlap with classes that represent special forms at runtime.

[danielknell]

Bug Report

A value annotated as a TypeForm is not considered to overlap with various clases used to represent special forms at runtime.

I have tested typing.NewType, types.GenericAlias, and types.UnionType. ¹

Given a variable defined as x: TypeForm[object] when using isinstance(x, typing.NewType) (or any of the above types), the truthy branch that follows is considered unreachable.

This does not happen with isinstance(x, type) though.

To Reproduce

from typing_extensions import TypeForm
import typing
import types

Y = typing.NewType("Y", int)

x1: TypeForm[object] = Y  # no issue, `Y` is a valid type expression

if isinstance(x1, typing.NewType):
    print("new type") # error: Statement is unreachable  [unreachable]
    
x2: TypeForm[object] = list[int]  # no issue, `list[int]` is a valid type expression

if isinstance(x2, types.GenericAlias):
    print("generic alias") # error: Statement is unreachable  [unreachable]
    
x3: TypeForm[object] = int | str  # no issue, `int | str` is a valid type expression

if isinstance(x3, types.UnionType):
    print("generic alias") # error: Statement is unreachable  [unreachable]
    
x4: TypeForm[object] = int  # no issue, `int` is a valid type expression

if isinstance(x4, type):
    print("generic alias") # no issue, code is reachable

https://mypy-play.net/?mypy=master&python=3.14&flags=warn-unreachable&gist=ea667f1dabb249a088448f23252c92c1

Expected Behavior

The code within the branches to be considered reachable.

Actual Behavior

main.py:10: error: Statement is unreachable  [unreachable]
main.py:15: error: Statement is unreachable  [unreachable]
main.py:20: error: Statement is unreachable  [unreachable]
Found 3 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.19.1 and main branch
• Mypy command-line flags: --enable-incomplete-feature=TypeForm --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 1.14

Footnotes

1. I think think is all the publicly documented classes? ↩