Add tests for capture checking, per lampepfl#67

shared/src/test/scala/CCBehavior.scala

@@ -0,0 +1,52 @@
+import language.experimental.captureChecking
+
+import gears.async.AsyncOperations.*
+import gears.async.default.given
+import gears.async.{Async, AsyncSupport, Future, uninterruptible}
+
+import java.util.concurrent.CancellationException
+import scala.annotation.capability
+import scala.concurrent.duration.{Duration, DurationInt}
+import scala.util.Success
+import scala.util.boundary
+
+type Result[+T, +E] = Either[E, T]
+object Result:
+  @capability opaque type Label[-T, -E] = boundary.Label[Result[T, E]]
+  // ^ doesn't work?
+
+  def apply[T, E](body: Label[T, E] ?=> T): Result[T, E] =
+    boundary(Right(body))
+
+  extension [U, E](r: Result[U, E]^)(using Label[Nothing, E]^)
+    def ok: U = r match
+      case Left(value)  => boundary.break(Left(value))
+      case Right(value) => value
+
+class CaptureCheckingBehavior extends munit.FunSuite:
+  import Result.*
+
+  test("good") {
+    // don't do this in real code! capturing Async.blocking's Async context across functions is hard to track
+    Async.blocking: async ?=>
+      def good1[T, E](frs: List[Future[Result[T, E]]]): Future[Result[List[T], E]]^{async} =
+        Future: 
+          Result: 
+            frs.map(_.await.ok)
+
+      def good2[T, E](rf: Result[Future[T], E]): Future[Result[T, E]]^{async} =
+        Future:
+          Result:
+            rf.ok.await // OK, Future argument has type Result[T]
+
+      def useless4[T, E](fr: Future[Result[T, E]]) =
+        fr.await.map(Future(_))
+  }
+
+  // test("bad") {
+  //   Async.blocking: async ?=>
+  //     def fail3[T, E](fr: Future[Result[T, E]]): Result[Future[T]^{async}, E] =
+  //       Result: label ?=>
+  //         Future: fut ?=>
+  //           fr.await.ok // error, escaping label from Result
+  // }