I heard back from both Samuel Colvin (Pydantic) and Sebastián Ramírez (FastAPI).  They said neither of them use update_wrapper with partial objects.  They also took a peek in a bunch of other projects (FastAPI, Typer, SQLModel, Asyncer, SQLAlchemy, Trio, and AnyIO) and nobody was doing it.

So honestly it seems like nobody (but me!) calls update_wrapper on partial objects, and we can just fix it.

Graham, any final thoughts before we start pulling levers and merging PRs?  For now I just want to fix this bug.  I'm in favor of re-engineering the relevant objects so they write their own __signature__ objects, so inspect.Signature doesn't have to understand the internals of objects from other modules.  But maybe we save that for another day.