Constrained `TypeVar` causes issues in type narrowing in attributes

[bzoracler]

Bug Report

A constrained typing.TypeVar object's attributes does not appear to type-narrow properly.

To Reproduce & Actual Behaviour

In the following example, an AST node transformation is trying to unravel the contents of an if typing.TYPE_CHECKING block into the owning namespace:

import ast
from typing import TypeVar

NamespaceNodeT = TypeVar("NamespaceNodeT", ast.Module, ast.ClassDef)

def unravel_TYPE_CHECKING(node: NamespaceNodeT) -> None:
    
    body_stmt: ast.stmt
    for body_stmt in list(node.body):
        if isinstance(body_stmt, ast.If) and (
            "TYPE_CHECKING" in ast.unparse(body_stmt.test)
        ):
            reveal_type(body_stmt)  # note: Revealed type is "_ast.If"
            reveal_type(body_stmt.body)  # note: Revealed type is "builtins.list[_ast.stmt]"
            if_idx: int = node.body.index(body_stmt)
            node.body[:] = [
                *node.body[:if_idx],
                *body_stmt.body,  # error: "stmt" has no attribute "body"  [attr-defined]
                *body_stmt.orelse,  # error: "stmt" has no attribute "orelse"  [attr-defined]
                *node.body[if_idx + 1 :],
            ]

The reveal_type lines see body_stmt as properly type-narrowed, but something else causes an issue when trying to use body_stmt's attributes?

Expected Behavior

I expected node: NamespaceNodeT to behave the same as node: ast.Module | ast.ClassDef, which causes no issues.

Your Environment

• Mypy version used: 0.991 and master
• Mypy command-line flags: (default)
• Mypy configuration options from mypy.ini (and other config files): (default)
• Python version used: 3.10

See mypy-play example.