Generalize the implementation of `SemanticDeclAvailabilityRequest` in
preparation for adding a new case to `SemanticDeclAvailability`. Use the
centralized availability constraint query instead of implementing a bespoke
algorithm for gathering constraints. Simplify `SemanticDeclAvailability` by
removing a case that is no longer relevant.
Part of rdar://138441307.
Extract the tests that verify back deployment of the standard library function
`diagnoseUnavailableCodeReached()` into a separate, multi-platform test.
Using availability domains, reimplement the algorithm that determines whether a
declaration is unavailable at runtime. The new algorithm takes ABI compatible
platforms into account, ensuring that declarations that are available on iOS do
not get treated as unreachable at runtime when compiling for visionOS.
Resolves rdar://116742214.
It should no longer be necessary to provide an `@_alwaysEmitIntoClient` version
of `_diagnoseUnavailableCodeReached()`. This workaround was originally added to
provide compatibility with projects that were misconfigured to compile against
a newer stdlib but link against an older one.
Resolves rdar://119892482.
The declarations in a zippered macOS library may be referenced by clients that
build either for macOS or for macCatalyst. It is therefore inappropriate to
stub functions in zippered libraries as "unreachable" when they are only
unavailable on macOS. Making this logic correct is a larger project, so for now
just disable stubbing when there is a target variant.
Resolves rdar://125371621
Functions that are marked `obsolete` may still be invoked at runtime so they
should not be stubbed when `-unavailable-decl-optimization=stub` is specified.
When determining whether a declaration should be considered unavailable at
runtime, ignore `@available` attributes for application extension platforms but
continue searching for other `@available` attributes that might still make the
declaration unavailable. This ensures corner cases like these are handled:
```
// Dubious, but allowed
@available(macOS, unavailable)
@available(macOSApplicationExtension, unavailable)
public func doublyUnavailableOnMacOSFunc() {}
// Expresses an uncommon, but valid constraint
@available(macCatalyst, unavailable)
@available(iOSApplicationExtension, unavailable)
public func confusingDiamondAvailabilityInheritanceFunc() {}
```
Unavailable decl optimization is meant to optimize declarations that are
unreachable at runtime. A declaration that is marked unavailable in app
extensions (e.g. `@available(macOSApplicationExtension, unavailable)`) may be
reachable by non-extension processes so it cannot be safely optimized.
Resolves rdar://122924862
When the deployment target is Swift 5.9 aligned or higher, we should not need
to call `_diagnoseUnavailableCodeReached()` via a back-deployment thunk since
the function will always be available in the standard library.
Resolves rdar://121878128
