A substitution map contains conformances, and a conformance can contain
a substitution map. This will always be a DAG, but not a tree, and to
avoid exponential blowup in certain edge cases, let's cache the work to
avoid visiting the same substitution map repeatedly, if multiple
conformances refer to the same substitution map.
Tracking seen declarations and substitution maps only detects the
situation where the opaque type's underlying type contains itself
with the same substitution map. However, it is also possible to
recurse with a different substitution map.
In general termination is undecidable with a problem like this,
so instead of trying to catch cycles, just impose a termination
limit.
This converts a stack overflow into an assertion, which is still
not ideal; we should really diagnose something instead. But this
is a first step.
Substitution of a pack expansion type may now produce a pack type.
We immediately expand that pack when transforming a tuple, a function
parameter, or a pack.
I had to duplicate the component-wise transformation logic in the
simplifyType transform, which I'm not pleased about, but a little
code duplication seemed a lot better than trying to unify the code
in two very different places.
I think we're very close to being able to assert that pack expansion
shapes are either pack archetypes or pack parameters; unfortunately,
the pack matchers intentionally produce expansions of packs, and I
didn't want to add that to an already-large patch.
There are a lot of problems caused by our highly-abstract substitution
subsystem. Most of them would be solved by a more semantic / holistic
understanding of the active transformation, but that's difficult to do
because we just pass around function_refs. The first step in fixing
that is to pass around a better currency type. For now, it can just
hold the function_refs (and the SubstOptions).
I've set it up so that the places that just apply SubstitutionMaps
are constructing the IFS in a standard way; that should make it easy
to change those places in the future.
