Add sealed refs test

Author: odersky

tests/neg-custom-args/captures/sealed-refs.scala

@@ -0,0 +1,42 @@
+class Ref[sealed A](init: A):
+  this: Ref[A]^ =>
+  private var x: A = init
+  def get: A = x
+  def set(x: A): Unit = this.x = x
+
+class It[X]:
+  this: It[X]^ =>
+
+def f1[B1](x: B1, next: B1 -> B1) =
+  var r = x    // ok
+  r = next(x)
+  r
+
+def f2[B2](x: B2, next: B2 -> B2) =
+  val r = Ref[B2](x)    // error
+  r.set(next(x))
+  r.get
+
+def g[sealed B](x: B, next: B -> B) =
+  val r = Ref[B](x)     // ok
+  r.set(next(x))
+  r.get
+
+import annotation.unchecked.uncheckedCaptures
+
+def h[B](x: B, next: B -> B) =
+  val r = Ref[B @uncheckedCaptures](x)  // ok
+  r.set(next(x))
+  r.get
+
+def f3[B](x: B, next: B -> B) =
+  val r: Ref[B^{cap[f3]}] = Ref[B^{cap[f3]}](x)    // error
+  r.set(next(x))
+  val y = r.get
+  ()
+
+def f4[B](x: B, next: B -> B) =
+  val r: Ref[B]^{cap[f4]} = Ref[B](x)    // error
+  r.set(next(x))
+  val y = r.get
+  ()