The current type inference scheme is such that we first introduce the
types for annotation functions, then check their bodies without
additional re-generalization. As part of generalization, we also perform
occurs checks to fix-up recursive tag unions.
However, type annotations can contain type inference variables that are
neither part of the generalization scheme, nor are re-generalized later
on, and in fact end up forming a closure of a recursive type. If we do
not catch and break such closures into recursive types, things go bad
soon after in later stages of the compiler.
To deal with this, re-introduce the values of recursive values after we
check their definitions, forcing an occurs check. This introduction is
benign because we already generalized appropriate type variables anyway.
Though, the introduction is somewhat unnecessary, and I have ideas on
how to make all of this simpler and more performant. That will come in
the future.
When constraining a recursive function like
```
f : _ -> {}
f : \_ -> f {}
```
our first step is to solve the value type of `f` relative to its
annotation. We have to be careful that the inference variable in the
signature of `f` is not generalized until after the body of `f` is
solved. Otherwise, we end up admitting polymorphic recursion.
With fixpoint-fixing, we don't want to re-unify type variables that were
just fixed, because doing so may change their shapes in ways that we
explicitly just set them up not to be changed (as fixpoint-fixing
clobbers type variable contents).
However, this restriction need only apply when we re-unify two type
variables that were both involved in the same fixpoint-fixing cycle. If
we have a type variable T that was involved in fixpoint-fixing, and we
unify it with U that wasn't, we know that the $U \notin \bar{T}$, where
$\bar{T}$ is the recursive closure of T. In these cases, we do want to
permit the usual in-band unification of $T \sim U$.
