As documented here, simple stubs/expectations do not stash or restore existing method defs.  This is OK for test doubles (the original case @xaviershay had in mind for simple stubs) but not for partial mocks (as will be used with this feature).  Can you add some specs (which should fail) about the resetting on a partial mocks, and then fix it?  I suggested one possible fix [here](https://github.com/rspec/rspec-mocks/pull/404#discussion_r5767427):

> In #399 we'd like to use the new simple stub stuff, but I guess a simple stub on a partial mock is a "slightly less simple stub" since it needs to be reset properly.  I think that should be taken care of as part of that ticket since it's not needed with what we currently have.  Here's an idea for how to do that: In `TestDouble#__build_mock_proxy` it will instantiate a subclass of `Proxy` that overrides `add_simple_stub` to pass a `support_reset: false` flag to `add_simple_stub`.  For other proxies (e.g. for a partial mock) it'll pass true for this flag, and based on the flag it'll either do `configure_method` or `define_proxy_method; @needs_restoration = false`.  I think that'll work.