to correctly handle generalized protocol requirements.
The major missing pieces here are that the conformance search
algorithms in both the AST (type substitution) and IRGen
(witness table reference emission) need to be rewritten to
back-track requirement sources, and the AST needs to actually
represent this stuff in NormalProtocolConformances instead
of just doing ???.
The new generality isn't tested yet; I'm looking into that,
but I wanted to get the abstractions in place first.
The list of directly inherited protocols of a ProtocolDecl is already
encoded in the requirement signature, as conformance constraints where
the subject is Self. Gather the list from there rather than separately
computing/storing the list of "inherited protocols".
There's a bit of a hack to deal with generic typealiases, but
overall this makes things more logical.
This is the last big refactoring before we can allow constrained
extensions to make generic parameters concrete. All that remains
is a small set of changes to SIL type lowering, and retooling
some diagnostics in Sema.
Split up parsing of typealias and associatedtype, including dropping a
now unneeded ParseDeclOptions flag.
Then made typealias in a protocol valid, and act like you would
hope for protocol conformance purposes (i.e. as an alias possibly
involved in the types of other func/var conformances, not as a hidden
generic param in itself).
Also added support for simple type aliases in generic constraints. Aliases
to simple (non-sugared) archetype types (and also - trivially - aliases to
concrete types) can now be part of same-type constraints.
The strategy here is to add type aliases to the tree of
PotentialArchetypes, and if they are an alias to an archetype, also to
immediately find the real associated type and set it as the
representative for the PA. Thus the typealias PA node becomes just a
shortcut farther down into the tree for purposes of lookup and
generating same type requirements.
Then the typealias PA nodes need to be explicitly skipped when walking
the tree for building archetype types and other types of requirements,
in order to keep from getting extra out-of-order archetypes/witness
markers of the real associated type inserted where the typealias is
defined.
Any constraint with a typealias more complex than pointing to a single
nested associated type (e.g. `typealias T = A.B.C.D`), will now get a
specialized diagnoses.
This will be used to help IRGen record protocol requirements
with resilient default implementations in protocol metadata.
To enable testing before all the Sema support is in place, this
patch adds SIL parser, printer and verifier support for default
witness tables.
For now, SILGen emits empty default witness tables for protocol
declarations in resilient modules, and IRGen ignores them when
emitting protocol metadata.
