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.
If a specialization of an ability member has a lambda set that is not
reflected in the unspecialized lambda sets of the member's prototype
signature, then the specialization lambda set is deemed to be immaterial
to the specialization lambda set mapping, and we don't need to associate
it with a particular region from the prototype signature.
This can happen when an opaque contains functions that are some specific
than the generalized prototype signature; for example, when we are
defining a custom impl for an opaque with functions.
Addresses a bug found in 8c3158c3e0
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 $T \sim U$.
When we attempt to bind a type argument to an ability in an alias/opaque
instantiation, we create a fresh flex var to represent satisfaction of
the ability, and then unify the type argument with that flex var.
Previously, we did not register this fresh var in the appropriate rank
pool.
Usually this is not a problem; however, our generalization algorithm is
such that we skip adjusting the rank of redundant variables. Redundant
variables are those that are in the same unification tree, but are not
the root of the unification trees.
This means that if such a flex able var becomes the root of a
unification tree with the type argument, and the type argument is itself
generalized, we will have missed generalization of the argument.
The fix is simple - make sure to register the flex able var into the
appropriate rank pool.
Closes#4408
Prior to this commit, we emplace type variables into `Index<TypeTag>`
only for translated top-level types. However, we need to be careful to
do this emplacement for nested types as well! This patch ensures we do,
but immediately emplacing the destination variable of a type when we
allocate a variable for it.
We're now reaching the steady state we want to be closert to - when a
type is translated to a variable, emplace the variable we created for it
in the type index, so that types are never converted again!
