More targeted handling of overriding checks against CapSet^

Author: odersky

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

@@ -1585,9 +1585,18 @@ class CheckCaptures extends Recheck, SymTransformer:
             memberTp match
               case TypeAlias(_) =>
                 otherTp match
-                  case otherTp: RealTypeBounds
-                  if otherTp.hi.isBoxedCapturing || otherTp.lo.isBoxedCapturing =>
-                    Some((memberTp, otherTp.unboxed))
+                  case otherTp: RealTypeBounds =>
+                    if otherTp.hi.isBoxedCapturing || otherTp.lo.isBoxedCapturing then
+                      Some((memberTp, otherTp.unboxed))
+                    else otherTp.hi match
+                      case hi @ CapturingType(parent: TypeRef, refs)
+                      if parent.symbol == defn.Caps_CapSet && refs.isUniversal =>
+                        Some((
+                          memberTp,
+                          otherTp.derivedTypeBounds(
+                            otherTp.lo,
+                            hi.derivedCapturingType(parent, root.Fresh().singletonCaptureSet))))
+                      case _ => None
                   case _ => None
               case _ => None
           else memberTp match

compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala

@@ -214,15 +214,14 @@ object OverridingPairs:
     if member.isType then // intersection of bounds to refined types must be nonempty
       memberTp.bounds.hi.hasSameKindAs(otherTp.bounds.hi)
       && (
-        CCState.withCapAsRoot: // If upper bound is CapSet^ any capture set of lower bound is OK
-          (memberTp frozen_<:< otherTp)
-          || !member.owner.derivesFrom(other.owner)
-              && {
-                // if member and other come from independent classes or traits, their
-                // bounds must have non-empty-intersection
-                val jointBounds = (memberTp.bounds & otherTp.bounds).bounds
-                jointBounds.lo frozen_<:< jointBounds.hi
-              }
+        (memberTp frozen_<:< otherTp)
+        || !member.owner.derivesFrom(other.owner)
+            && {
+              // if member and other come from independent classes or traits, their
+              // bounds must have non-empty-intersection
+              val jointBounds = (memberTp.bounds & otherTp.bounds).bounds
+              jointBounds.lo frozen_<:< jointBounds.hi
+            }
       )
     else
       member.name.is(DefaultGetterName) // default getters are not checked for compatibility