Adapt types according to read-only status

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

@@ -70,14 +70,17 @@ class CCState:
 
   // ------ Iteration count of capture checking run
 
-  private var iterCount = 1
+  private var iterCount = 0
 
   def iterationId = iterCount
 
   def nextIteration[T](op: => T): T =
     iterCount += 1
     try op finally iterCount -= 1
 
+  def start(): Unit =
+    iterCount = 1
+
   // ------ Global counters -----------------------
 
   /** Next CaptureSet.Var id */

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

@@ -330,6 +330,12 @@ object Capabilities:
     final def isExclusive(using Context): Boolean =
       !isReadOnly && (isTerminalCapability || captureSetOfInfo.isExclusive)
 
+    /** Similar to isExlusive, but also includes capabilties with capture
+     *  set variables in their info whose status is still open.
+     */
+    final def maybeExclusive(using Context): Boolean =
+      !isReadOnly && (isTerminalCapability || captureSetOfInfo.maybeExclusive)
+
     final def isWellformed(using Context): Boolean = this match
       case self: CoreCapability => self.isTrackableRef
       case _ => true

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

@@ -22,11 +22,18 @@ private val Captures: Key[CaptureSet] = Key()
 def isCaptureChecking(using Context): Boolean =
   ctx.phaseId == Phases.checkCapturesPhase.id
 
-/** Are we at checkCaptures or Setup phase? */
+/** Are we in the CheckCaptures or Setup phase? */
 def isCaptureCheckingOrSetup(using Context): Boolean =
-  val ccId = Phases.checkCapturesPhase.id
-  val ctxId = ctx.phaseId
-  ctxId == ccId || ctxId == ccId - 1
+  val ccPhase = Phases.checkCapturesPhase
+  ccPhase.exists
+  && {
+    val ccId = ccPhase.id
+    val ctxId = ctx.phaseId
+    ctxId == ccId
+    || ctxId == ccId - 1 && ccState.iterationId > 0
+      // Note: just checking phase id is not enough since Setup would
+      // also be the phase after pattern matcher.
+  }
 
 /** A dependent function type with given arguments and result type
  *  TODO Move somewhere else where we treat all function type related ops together.