Tests for capture variable subtyping

tests/neg-custom-args/captures/capture-vars-subtyping.scala

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+import language.experimental.captureChecking
+import caps.*
+
+def test[C^] = 
+  val a: C = ???
+  val b: CapSet^{C^} = a
+  val c: C = b
+  val d: CapSet^{C^, c} = a
+
+// TODO: make "CapSet-ness" of type variables somehow contagious?
+// Then we don't have to spell out the bounds explicitly...
+def testTrans[C^, D >: CapSet <: C, E >: CapSet <: D, F >: C <: CapSet^] =
+  val d1: D = ???
+  val d2: CapSet^{D^} = d1
+  val d3: D = d2
+  val e1: E = ???
+  val e2: CapSet^{E^} = e1
+  val e3: E = e2
+  val d4: D = e1
+  val c1: C = d1
+  val c2: C = e1
+  val f1: F = c1
+  val d_e_f1: CapSet^{D^,E^,F^} = d1
+  val d_e_f2: CapSet^{D^,E^,F^} = e1
+  val d_e_f3: CapSet^{D^,E^,F^} = f1
+  val f2: F = d_e_f1
+  val c3: C = d_e_f1 // error
+  val c4: C = f1     // error
+  val e4: E = f1     // error
+  val e5: E = d1     // error
+
+
+trait A[+T]
+
+trait B[-C]
+
+def testCong[C^, D^] =
+  val a: A[C] = ???
+  val b: A[CapSet^{C^}] = a
+  val c: A[CapSet^{D^}] = a // error
+  val d: A[CapSet^{C^,D^}] = a
+  val e: A[C] = d // error
+  val f: B[C] = ???
+  val g: B[CapSet^{C^}] = f
+  val h: B[C] = g
+  val i: B[CapSet^{C^,D^}] = h // error
+  val j: B[C] = i