> Good sleuthing! So the bug is in _lru_cache?

I don't think it's a bug of lru_cache. It stores strong references for arguments and return value of the decorated function (which btw isn't documented).  I don't think it's possible to write efficient generic lru_cache that would just use weakrefs.

The problem is that because _tp_cache helper uses it, the lifetime of some typing classes becomes indefinite.  Which shouldn't be a problem for a typical user of typing, since most of the time it is used on the module level.

We have two potential solutions to this problem:

1. Add a weak-ref type cache and refactor _tp_cache to use it;

2. Cleanup lru-cache before/adter running each typing test.

[1] is the ideal solution, [2] should work good enough.