Add advanced test case for classifier capabilities

Author: bracevac

tests/neg-custom-args/captures/classifiers-secondclass.scala

@@ -0,0 +1,67 @@
+import language.experimental.captureChecking
+import language.experimental.separationChecking
+import caps.*
+
+// Test inspired by the "Gentrification Gone too Far?" paper
+object Levels:
+
+  trait Read extends Classifier, Capability
+  trait ReadWrite extends Classifier, Capability
+
+  trait File(val name: String):
+    val r: Read^
+    val rw: ReadWrite^
+    // operations guarded by boxed capability members
+    val read: () ->{r} Int
+    val write: Int ->{rw} Unit
+
+  object File:
+    def apply(s: String): File^ = new File(s) {
+      val r = new Read {}
+      val rw = new ReadWrite {}
+      val read = () =>
+        println(s"Reading from $name with capability $r")
+        42
+      val write = (i: Int) =>
+        println(s"Writing $i to $name with capability $rw")
+    }
+
+  // Unfortunately, we do not have @use lambdas yet
+  trait UseFunction[U]:
+    def apply(@use f: File^): U
+
+  def withFile[U](name: String)(block: UseFunction[U]): U = block(File(name)) // unrestricted use of files & other capabilities
+  def parReduce[U](xs: Seq[U])(op: (U, U) ->{cap.only[Read]} U): U = xs.reduce(op) // only Read-classified allowed
+
+  @main def test =
+    withFile("foo.txt"):
+      new UseFunction[Unit]:
+        def apply(@use f: File^): Unit =
+          f.read() // ok
+          parReduce(1 to 1000): (a, b) =>
+            a * b * f.read()    // ok
+          parReduce(1 to 1000): (a, b) => // error
+            f.write(42)         // the error stems from here
+            a + b + f.read()    // ok
+          f.write(42)           // ok, unrestricted access to file
+
+  def testMulti =
+    withFile("foo.txt"):
+      new UseFunction[Unit]:
+        def apply(@use f: File^): Unit =
+          withFile("bar.txt"):
+            new UseFunction[Unit]:
+              def apply(@use g: File^): Unit =
+                f.read() // ok
+                g.read() // ok
+                parReduce(1 to 1000): (a, b) =>
+                  a * b * f.read() + g.read() // ok
+                parReduce(1 to 1000): (a, b) => // error
+                  f.write(42)         // the error stems from here
+                  a + b + f.read() + g.read() // ok
+                parReduce(1 to 1000): (a, b) => // error
+                  g.write(42)         // the error stems from here
+                  0
+                f.write(42)           // ok, unrestricted access to file
+                g.write(42)           // ok, unrestricted access to file
+