A generic defn produces no Tast.fn, so the editor was told nothing had
been installed and nothing had gone wrong. eval now expands a redefined
generic name to its copies, and picks up any copy the running process
was never built with - which is how a redefined caller reaching a
generic at a new element type gets that copy built and loaded.
C-x C-e is the path that could really go stale, and did: it checks
against the live environment, so an expression naming a generic at an
unused type generated a copy that existed in no program and the thunk
called a symbol nothing defined. Marked and spliced.
There was no cache to invalidate. program_with_env builds a fresh env
every evaluation, so the instantiation cache cannot survive one; the
test pins that rather than inventing machinery for it.
A signature change reaches the session as a refusal about put!-i32, a
name the source does not contain. It now says which generic it is a
copy of, at which types, and that every copy changed together.
