Fix underlying problem of box mismatches

The neg tests are now pos tests. I kept the special error message for better diagnosis
if the mismatch arises elsewhere, and added a sentence that this could be an implementation
problem.

Author: odersky

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

@@ -1388,7 +1388,8 @@ object CaptureSet:
         trailing:
           val (boxed, unboxed) =
             if cs.tp1.isBoxedCapturing then (cs.tp1, cs.tp2) else (cs.tp2, cs.tp1)
-          i"${cs.tp1} does not conform to ${cs.tp2} because $boxed is boxed but $unboxed is not"
+          i"""${cs.tp1} does not conform to ${cs.tp2} because $boxed is boxed but $unboxed is not.
+             |This could be an error in the capture checker implementation."""
       case _ =>
         def why =
           val reasons = cs.elems.toList.collect:

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

@@ -550,10 +550,15 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             || !ctx.mode.is(Mode.CheckBoundsOrSelfType) && tp1.isAlwaysPure
             || parent1.isSingleton && refs1.elems.forall(parent1 eq _)
           then
+            def remainsBoxed1 = parent1.isBoxedCapturing || parent1.dealias.match
+              case parent1: TypeRef =>
+                parent1.superType.isBoxedCapturing
+                // When comparing a type parameter with boxed upper bound on the left
+                // we should not strip the box on the right. See i24543.scala.
+              case _ =>
+                false
             val tp2a =
-              if tp1.isBoxedCapturing && !parent1.isBoxedCapturing
-              then tp2.unboxed
-              else tp2
+              if tp1.isBoxedCapturing && !remainsBoxed1 then tp2.unboxed else tp2
             recur(parent1, tp2a)
           else thirdTry
         compareCapturing
@@ -2916,7 +2921,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     subc
     && (tp1.isBoxedCapturing == tp2.isBoxedCapturing
         || refs1.subCaptures(CaptureSet.EmptyOfBoxed(tp1, tp2), makeVarState()))
-        
+
   protected def logUndoAction(action: () => Unit) =
     undoLog += action
 

tests/neg-custom-args/captures/i24543.check

@@ -4,9 +4,9 @@ class Ref
 case class Box[T](elem: T)
 
 def h1[T <: Ref^](x: Box[T]): Unit =
-  val y: (Ref^, Int) = (x.elem, 1) // error
+  val y: (Ref^, Int) = (x.elem, 1) // was error
 
-def h2[T <: Ref^](x: List[T]): (Ref^, Int) = (x.head, 1) // error
+def h2[T <: Ref^](x: List[T]): (Ref^, Int) = (x.head, 1) // was error
 
 
 

tests/neg-custom-args/captures/i24543.scala renamed to tests/pos-custom-args/captures/i24543.scala

@@ -0,0 +1,9 @@
+import scala.language.experimental.captureChecking
+class Ref
+case class Box[+T](elem: T)
+
+def test1(a: Ref^): Unit =
+  def hh[T <: Ref^{a}](x: Box[T]): Unit =
+    val y: (Ref^{a}, Int) = (x.elem, 1)
+    val z = (x.elem, 1)
+    val _: (Ref^{a}, Int) = z