swift234

Model references to DynamicSelf methods on existentials by opening the existential.

Introduce a new expression kind, OpenExistentialExpr, that "opens" up
an existential value into a value of a fresh archetype type that
represents the dynamic type of the existential. That value can be
referenced (via an OpaqueValueExpr) within the within the
subexpression of OpenExistentialExpr. For example, a call to a
DynamicSelf method on an existential looks something like this:

        (open_existential_expr implicit type='P'
          (opaque_value_expr implicit type='opened P' @ 0x7fd95207c290
          unique)
          (load_expr implicit type='P'
            (declref_expr type='@lvalue P' decl=t.(file).func
          decl.p@t.swift:5:37 specialized=no))
          (erasure_expr implicit type='P'
            (call_expr type='opened P'
              (archetype_member_ref_expr type='() -> opened P'
          decl=t.(file).P.f@t.swift:2:8 [with Self=opened P]
                (opaque_value_expr implicit type='opened P' @
          0x7fd95207c290 unique))
              (tuple_expr type='()')))))

Note that we're using archetype_member_ref_expr rather than
existential_member_ref_expr, because the call is operating on the
opaque_value_expr of archetype type. The outer erasure turns the
archetype value back into an existential value.

The SILGen side of this is somewhat incomplete; we're using
project_existential[_ref] to open the existential, which is almost
correct: it gives us access to the value as an archetype, but IRGen
doesn't know to treat the archetype type as a fresh archetype whose
conformances come from the existential. Additionally, the output of
the opened type is not properly parsable. I'll fix this in follow-on
commits.

Finally, the type checker very narrowly introduces support for
OpenExistentialExpr as it pertains to DynamicSelf. However, this can
generalize to support all accesses into existentials, eliminating the
need for ExistentialMemberRef and ExistentialSubscript in the AST and
protocol_method in SIL, as well as enabling more advanced existential
features should we want them later.



Swift SVN r13740