Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.
Adjust macro usages accordingly.
Rather than blanket-disabling concurrency tests when we aren't using a
just-built concurrency library, enable them whenever we have a
suitable concurrency runtime, either just-built, in the OS, or via the
back-deployment libraries.
The `Task` type has oscillated somewhat from being purely a namespace,
to having instances that are used (albeit rarely), back to purely
being a namespace that isn't used for all that many names. Many of the
names that used to be on Task have already been moved out, e.g., for
creating new detached tasks, creating new task groups, adding
cancellation handlers, etc.
Collapse `Task.Handle<Success, Failure>` into `Task<Success, Failure>`.
`Task.Handle` is the type that is most frequently referenced in the
concurrency library, so giving it the short name `Task` is most
appropriate. Replace the top-level async/detach functions with a
`Task` initializer and `Task.detached`, respectively.
The `Task` type can still act as a namespace for static operations
such as, e.g., `Task.isCancelled`. Do this with an extension of the
form:
extension Task where Success == Never, Failure == Never { ... }
We've been accruing a number of compatibility shims. Move them all
into their own source file, deprecate them, and make them
always-emit-into-client so they don't have any ABI impact.
This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.
rdar://75850003
