During the unspecialized lambda set compaction procedure, we might end
up trying to merge too many disjoint variables during unspecialized
lambda unification. Avoid doing so, by checking if we're in the
compaction procedure.
Even if there are no changes to alias arguments, and no new variables were
introduced, we may still need to unify the "actual types" of the alias or opaque!
The unification is not necessary from a types perspective (and in fact, we may want
to disable it for `roc check` later on), but it is necessary for the monomorphizer,
which expects identical types to be reflected in the same variable.
As a concrete example, consider the unification of two opaques
P := [Zero, Succ P]
(@P (Succ n)) ~ (@P (Succ o))
`P` has no arguments, and unification of the surface of `P` introduces nothing new.
But if we do not unify the types of `n` and `o`, which are recursion variables, they
will remain disjoint! Currently, the implication of this is that they will be seen
to have separate recursive memory layouts in the monomorphizer - which is no good
for our compilation model.
Closes#3653
It's very possible to unify two variables without their actual variable
numbers having been merged in the unification forest. We might want to
do that in the future, but it's not necessarily true today. For example
two concrete constructors `{}` and `{}` are unified by their contents,
but the variables are not necessarily merged afterward.
