Add to test

Author: odersky

tests/neg-custom-args/captures/ro-mut-conformance.check

@@ -1,10 +1,10 @@
--- Error: tests/neg-custom-args/captures/ro-mut-conformance.scala:8:4 --------------------------------------------------
-8 |  a.set(42)  // error
-  |  ^^^^^
-  |  cannot call update method set from (a : Ref),
-  |  since its capture set {a} is read-only
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/ro-mut-conformance.scala:10:21 ---------------------------
-10 |  val t: Ref^{cap} = a  // error
+-- Error: tests/neg-custom-args/captures/ro-mut-conformance.scala:10:4 -------------------------------------------------
+10 |  a.set(42)  // error
+   |  ^^^^^
+   |  cannot call update method set from (a : Ref),
+   |  since its capture set {a} is read-only
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/ro-mut-conformance.scala:12:21 ---------------------------
+12 |  val t: Ref^{cap} = a  // error
    |                     ^
    |                     Found:    (a : Ref)
    |                     Required: Ref^

tests/neg-custom-args/captures/ro-mut-conformance.scala

@@ -1,11 +1,15 @@
 import language.experimental.captureChecking
 import caps.*
-class Ref extends Mutable:
+trait IRef:
+  def get: Int
+class Ref extends IRef, Mutable:
   private var _data: Int = 0
   def get: Int = _data
   update def set(x: Int): Unit = _data = x
 def test1(a: Ref^{cap.rd}): Unit =
   a.set(42)  // error
 def test2(a: Ref^{cap.rd}): Unit =
   val t: Ref^{cap} = a  // error
+  def b: Ref^{cap.rd} = ???
+  val i: IRef^{cap} = b // ok, no added privilege from `cap` on an IRef
   t.set(42)