The model for associated types hasn't been fully worked-out for
noncopyable generics, but there is some support already that is being
used by the stdlib for an internal-only (and rather cursed) protocol
`_Pointer` to support `UnsafePointer`, etc.
This patch gates the existing experimental support for associated types
behind a feature flag. This flag doesn't emit feature-guards in
interfaces, since support for it is tied closely to NoncopyableGenerics
and has been there from its early days.
When a NoncopyableGenericsMismatch happens between the compiler and
stdlib, allow the compiler to rebuild the stdlib from its interface
instead of exiting with an error.
We want extensions to introduce default Copyable/Escapable just like
other generic contexts, so that once Optional adopts ~Copyable,
an `extension Optional` actually adds `Wrapped: Copyable` by default.
Follow the feature flag convention for capitalization and be
consistent with the related NoncopyableGenerics feature.
This is a new feature that no wild Swift code has used it yet:
commit e99ce1cc5d
Author: Kavon Farvardin <kfarvardin@apple.com>
Date: Tue Dec 5 23:25:09 2023
[NCGenerics] add `~Escapable`
Basic implementation of `~Escapable` in the type system.
