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

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

[^1]: I think think is all the publicly documented classes?

Der Bericht nennt weder eine Quelldatei noch einen vorhandenen Test. Führe zunächst den mitgelieferten Reproducer mit --enable-incomplete-feature=TypeForm und --warn-unreachable aus und verfolge anschließend die Behandlung von Überschneidungen bei TypeForm für typing.NewType, types.GenericAlias und types.UnionType. Erledigt ist die Aufgabe, wenn diese Zweige als erreichbar betrachtet werden und ein Regressionstest die Fälle abdeckt.