Don't follow opaque aliases when transforming sym info for cc

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

@@ -116,7 +116,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
    *  convert it to be boxed.
    */
   private def box(tp: Type)(using Context): Type =
-    def recur(tp: Type): Type = tp.dealiasKeepAnnots match
+    def recur(tp: Type): Type = tp.dealiasKeepAnnotsAndOpaques match
       case tp @ CapturingType(parent, refs) =>
         if tp.isBoxed then tp else tp.boxed
       case tp @ AnnotatedType(parent, ann) =>
@@ -574,7 +574,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
         if sym.isClass then
           !sym.isPureClass && sym != defn.AnyClass
         else
-          val tp1 = tp.dealiasKeepAnnots
+          val tp1 = tp.dealiasKeepAnnotsAndOpaques
           if tp1 ne tp then needsVariable(tp1)
           else instanceCanBeImpure(tp1)
       case tp: (RefinedOrRecType | MatchType) =>
@@ -640,7 +640,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
       def maybeAdd(target: Type, fallback: Type) =
         if needsVariable(target) then CapturingType(target, addedSet(target))
         else fallback
-      val dealiased = tp.dealiasKeepAnnots
+      val dealiased = tp.dealiasKeepAnnotsAndOpaques
       if dealiased ne tp then
         val transformed = transformInferredType(dealiased)
         maybeAdd(transformed, if transformed ne dealiased then transformed else tp)

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

@@ -992,18 +992,18 @@ object Denotations {
       if (symbol == NoSymbol) symbol.toString
       else s"<SingleDenotation of type $infoOrCompleter>"
 
-    def definedPeriodsString: String = {
+    /** Show all defined periods and the info of the denotation at each */
+    def definedPeriodsString(using Context): String = {
       var sb = new StringBuilder()
       var cur = this
       var cnt = 0
-      while ({
-        sb.append(" " + cur.validFor)
+      while
+        sb.append(i" ${cur.validFor.toString}:${cur.infoOrCompleter}")
         cur = cur.nextInRun
         cnt += 1
         if (cnt > MaxPossiblePhaseId) { sb.append(" ..."); cur = this }
         cur ne this
-      })
-      ()
+      do ()
       sb.toString
     }
 

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

@@ -1460,13 +1460,13 @@ object Types {
       case _ => this
     }
 
-    /** Follow aliases and dereferences LazyRefs, annotated types and instantiated
+    /** Follow aliases and dereference LazyRefs, annotated types and instantiated
      *  TypeVars until type is no longer alias type, annotated type, LazyRef,
      *  or instantiated type variable.
      */
     final def dealias(using Context): Type = dealias1(keepNever, keepOpaques = false)
 
-    /** Follow aliases and dereferences LazyRefs and instantiated TypeVars until type
+    /** Follow aliases and dereference LazyRefs and instantiated TypeVars until type
      *  is no longer alias type, LazyRef, or instantiated type variable.
      *  Goes through annotated types and rewraps annotations on the result.
      */
@@ -1475,12 +1475,30 @@ object Types {
     /** Like `dealiasKeepAnnots`, but keeps only refining annotations */
     final def dealiasKeepRefiningAnnots(using Context): Type = dealias1(keepIfRefining, keepOpaques = false)
 
-    /** Follow non-opaque aliases and dereferences LazyRefs, annotated types and instantiated
-     *  TypeVars until type is no longer alias type, annotated type, LazyRef,
-     *  or instantiated type variable.
+    /** Like dealias, but does not follow aliases if symbol is Opaque. This is
+     *  necessary if we want to look at the info of a symbol containing opaque
+     *  type aliases but pretend "it's from the outside". For instance, consider:
+     *
+     *    opaque type IArray[T] = Array[? <: T]
+     *    object IArray:
+     *      def head[T](xs: IArray[T]): T = ???
+     *
+     *  If we dealias types in the info of `head`, those types appear with prefix
+     *  IArray.this, where IArray's self type is `IArray { type IArray[T] = Array[? <: T] }`.
+     *  Hence, if we see IArray it will appear as an alias of [T] =>> Array[? <: T].
+     *  But if we want to see the type from the outside of object IArray we need to
+     *  suppress this dealiasing. A test case where this matters is i18909.scala.
+     *  Here, we dealias symbol infos at the start of capture checking in operation `fluidify`.
+     *  We have to be careful not to accidentally reveal opaque aliases when doing so.
      */
     final def dealiasKeepOpaques(using Context): Type = dealias1(keepNever, keepOpaques = true)
 
+    /** Like dealiasKeepAnnots, but does not follow opaque aliases. See `dealiasKeepOpaques`
+     *  for why this is sometimes necessary.
+     */
+    final def dealiasKeepAnnotsAndOpaques(using Context): Type =
+      dealias1(keepAlways, keepOpaques = true)
+
     /** Approximate this type with a type that does not contain skolem types. */
     final def deskolemized(using Context): Type =
       val deskolemizer = new ApproximatingTypeMap {
@@ -5351,6 +5369,8 @@ object Types {
       case that: AliasingBounds => this.isTypeAlias == that.isTypeAlias && alias.eq(that.alias)
       case _ => false
     }
+
+    override def toString = s"${getClass.getSimpleName}($alias)"
   }
 
   /**    = T

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1053,9 +1053,9 @@ class TreeUnpickler(reader: TastyReader,
         else EmptyValDef
       cls.setNoInitsFlags(parentsKind(parents), bodyFlags)
       cls.info = ClassInfo(
-        cls.owner.thisType, cls, parentTypes, cls.unforcedDecls,
-        selfInfo = if (self.isEmpty) NoType else self.tpt.tpe)
-        .integrateOpaqueMembers
+          cls.owner.thisType, cls, parentTypes, cls.unforcedDecls,
+          selfInfo = if (self.isEmpty) NoType else self.tpt.tpe
+        ).integrateOpaqueMembers
       val constr = readIndexedDef().asInstanceOf[DefDef]
       val mappedParents: LazyTreeList =
         if parents.exists(_.isInstanceOf[InferredTypeTree]) then

tests/pos/i18909.scala

@@ -0,0 +1,4 @@
+import language.experimental.captureChecking
+def foo(): Unit =
+  val r1: IArray[String] = ???
+  val r2: String = IArray.head(r1)