Recompute denotations when computing ofInfo of reach capabilities

Fixes scala#23582

But it is quite a hack. We should try to find a more principled solution.

Author: odersky

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

@@ -1573,6 +1573,10 @@ object CaptureSet:
   /** The capture set of the type underlying the capability `c` */
   def ofInfo(c: Capability)(using Context): CaptureSet = c match
     case Reach(c1) =>
+      c1 match
+        case c1: TermRef => c1.safeRecomputeDenot()
+          // following widen would be wrong otherwise
+        case _ =>
       c1.widen.computeDeepCaptureSet(includeTypevars = true)
         .showing(i"Deep capture set of $c: ${c1.widen} = ${result}", capt)
     case Restricted(c1, cls) =>

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -2601,6 +2601,10 @@ object Types extends TypeUtils {
     def recomputeDenot()(using Context): Unit =
       setDenot(memberDenot(name, allowPrivate = !symbol.exists || symbol.is(Private)))
 
+    def safeRecomputeDenot()(using Context): Unit =
+      val newDenot = memberDenot(name, allowPrivate = !symbol.exists || symbol.is(Private))
+      if newDenot.exists then setDenot(newDenot)
+
     private def setDenot(denot: Denotation)(using Context): Unit = {
       if ctx.base.checkNoDoubleBindings then
         checkSymAssign(denot.symbol)

tests/neg-custom-args/captures/i23582.check

@@ -0,0 +1,13 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23582.scala:27:26 ---------------------------------------
+27 |    parReduce(1 to 1000): (x, y) => // error
+   |                          ^
+   |Found:    (x: Int, y: Int) ->{write, read} Int
+   |Required: (Int, Int) ->{cap.only[Read]} Int
+   |
+   |Note that capability write is not included in capture set {cap.only[Read]}.
+   |
+   |where:    cap is a fresh root capability created in method test when checking argument to parameter op of method parReduce
+28 |      write(x)
+29 |      x + y + read()
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i23582.scala

@@ -0,0 +1,30 @@
+import caps.*
+object Levels:
+
+  trait Read extends Classifier, SharedCapability
+  trait ReadWrite extends Classifier, SharedCapability
+
+  class Box[T](acc: T):
+    val access: T = acc
+
+  def parReduce(xs: Seq[Int])(op: (Int, Int) ->{cap.only[Read]} Int): Int = xs.reduce(op)
+
+  @main def test =
+    val r: Box[Read^] = ???
+    val rw: Box[ReadWrite^] = ???
+    val read: () ->{r.access*} Int = ???
+    val write: Int ->{rw.access*} Unit = ???
+    val checkRead: () ->{cap.only[Read]} Int = read
+
+    //read() // causes error with and without the println below
+    parReduce(1 to 1000): (x, y) =>
+      //println(r.access) // ok only if this is uncommented
+      read() + read() // should be ok
+
+    parReduce(1 to 1000): (x, y) =>
+      x + y + read() // should be ok
+
+    parReduce(1 to 1000): (x, y) => // error
+      write(x)
+      x + y + read()
+