We want a conditionally-copyable type to still be classified as trivial in cases
where it's bitwise-copyable, has a trivial deinit, and is Copyable. The previous
implementation here only checked at the declaration level whether a type was
Copyable or not; get a more accurate answer by consulting the combination
of information in the substituted type and abstraction pattern we have
available during type lowering so that we classify definitely-copyable substitutions
of a conditionally-copyable type as trivial. Should fix rdar://123654553 and
rdar://123658878.
If we fail to build a generic signature (or requirement signature of a
protocol) because of a request cycle or because Knuth-Bendix completion
failed, we would create a placeholder signature with no requirements.
However in a move-only world, a completely unconstrained generic
parameter might generate spurious diagnostics when used in a copyable
way. For this reason, let's outfit these placeholder signatures with
a default set of conformance requirements to Copyable and Escapable.
We weren't diagnosing conflicts in PCT's like `Copyable & ~Copyable`,
instead deferring until that PCT was constrained to something like the
existential Self or a generic parameter, which then we'd diagnose.
But we should canonicalize PCT's such as `Copyable & Copyable` into
`Any`, which represents the empty composition. That's what the assert in
PCT::build is about.
We can't simply emit the desugared, expanded version of the requirements
because there's no way to pretty-print the type `some ~Copyable` when
the `~Copyable`'s get replaced with the absence of `Copyable`. We'd be
left with just `some _` or need to invent a new top type so we can write
`some Top`. Thus, it's best to simply reverse the expansion of default
requirements when emitting a swiftinterface file.
We already need to track the inverses separate from the members in a
ProtocolCompositionType, since inverses aren't real types. Thus, the
only purpose being served by InverseType is to be eliminated by
RequirementLowering when it appears in a conformance requirement.
Instead, we introduce separate type InverseRequirement just to keep
track of which inverses we encounter to facilitate cancelling-out
defaults and ensuring that the inverses are respected after running
the RequirementMachine.
