jinja_loader type bug

[Cody Scott]

I have

from jinja2 import ChoiceLoader, FileSystemLoader


app.jinja_loader = ChoiceLoader([
    FileSystemLoader(project_dir / 'templates/'),
    app.jinja_loader,
])

There is a mypy error

error: Incompatible types in assignment (expression has type "ChoiceLoader", variable has type "FileSystemLoader | None")  [assignment]
error: List item 1 has incompatible type "FileSystemLoader | None"; expected "BaseLoader"  [list-item]

There shouldn't be an error.

Environment:

• Python version: 3.12.1
• Flask version: 3.0.1