Also drop read-only versions of shared capabilities from separation checking

We do get sometimes capabilities like `async.rd` where `async` is shared. It
does not look straightforward to prevent that, so instead we count async.rd
as shared if async is.

Author: odersky

compiler/src/dotty/tools/dotc/cc/SepCheck.scala

@@ -638,7 +638,7 @@ class SepCheck(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
     val badParams = mutable.ListBuffer[Symbol]()
     def currentOwner = role.dclSym.orElse(ctx.owner)
     for hiddenRef <- refsToCheck.deduct(explicitRefs(tpe)) do
-      if !hiddenRef.isKnownClassifiedAs(defn.Caps_SharedCapability) then
+      if !hiddenRef.stripReadOnly.isKnownClassifiedAs(defn.Caps_SharedCapability) then
         hiddenRef.pathRoot match
           case ref: TermRef if ref.symbol != role.dclSym =>
             val refSym = ref.symbol
@@ -675,7 +675,7 @@ class SepCheck(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
     role match
       case _: TypeRole.Argument | _: TypeRole.Qualifier =>
         for ref <- refsToCheck do
-          if !ref.isKnownClassifiedAs(defn.Caps_SharedCapability) then
+          if !ref.stripReadOnly.isKnownClassifiedAs(defn.Caps_SharedCapability) then
             consumed.put(ref, pos)
       case _ =>
   end checkConsumedRefs

tests/pos-custom-args/captures/ro-array.scala

@@ -0,0 +1,22 @@
+import caps.*
+object Test
+
+class Arr[T: reflect.ClassTag](a: Async, len: Int) extends Mutable:
+  private val arr: Array[T] = new Array[T](len)
+  def get(i: Int): T = arr(i)
+  update def set(i: Int, x: T): Unit = arr(i) = x
+
+class Async extends SharedCapability
+
+def f[T](x: T): T & Pure = x.asInstanceOf[T & Pure]
+
+def test =
+  def x(async: Async): Arr[String]^{cap.rd} =
+    val y = Arr[String](async, 10)
+    for i <- 0 to 10 do
+      y.set(i, "A")
+    val z = f(y)
+    y
+
+
+