Print subcapturing steps in -explain traces

Author: odersky

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

@@ -343,7 +343,9 @@ object CaptureSet:
     opaque type Type = CaptureSet
     val OK: Type = Const(emptySet)
     def fail(cs: CaptureSet): Type = cs
-    extension (result: Type) def blocking: CaptureSet = result
+    extension (result: Type)
+      def blocking: CaptureSet = result
+      def show: String = if result == OK then "OK" else result.toString
 
   class VarState:
     private val elemsMap: util.EqHashMap[Var, Refs] = new util.EqHashMap

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

@@ -491,7 +491,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             // under -Ycheck. Test case is i7965.scala.
 
       case CapturingType(parent1, refs1, _) =>
-        if refs1.subCaptures(tp2.captureSet, frozenConstraint) == CaptureSet.CompareResult.OK then
+        if subCaptures(refs1, tp2.captureSet, frozenConstraint) == CaptureSet.CompareResult.OK then
           recur(parent1, tp2)
         else
           thirdTry
@@ -2396,7 +2396,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     case tp1: TypeVar if tp1.isInstantiated =>
       tp1.underlying & tp2
     case CapturingType(parent1, refs1, _) =>
-      if tp2.captureSet.subCaptures(refs1, frozenConstraint) == CaptureSet.CompareResult.OK then
+      if subCaptures(tp2.captureSet, refs1, frozenConstraint) == CaptureSet.CompareResult.OK then
         parent1 & tp2
       else
         tp1.derivedCapturingType(parent1 & tp2, refs1)
@@ -2462,6 +2462,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       false
   }
 
+  protected def subCaptures(refs1: CaptureSet, refs2: CaptureSet, frozen: Boolean)(using Context): CaptureSet.CompareResult.Type =
+    refs1.subCaptures(refs2, frozen)
+
   // ----------- Diagnostics --------------------------------------------------
 
   /** A hook for showing subtype traces. Overridden in ExplainingTypeComparer */
@@ -2730,6 +2733,7 @@ object TypeComparer {
     else res match
       case ClassInfo(_, cls, _, _, _) => cls.showLocated
       case bounds: TypeBounds => i"type bounds [$bounds]"
+      case CaptureSet.CompareResult.OK => "OK"
       case res: printing.Showable => res.show
       case _ => String.valueOf(res)
 
@@ -3059,5 +3063,10 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       super.addConstraint(param, bound, fromBelow)
     }
 
+  override def subCaptures(refs1: CaptureSet, refs2: CaptureSet, frozen: Boolean)(using Context): CaptureSet.CompareResult.Type =
+    traceIndented(i"subcaptures $refs1 <:< $refs2 ${if frozen then "frozen" else ""}") {
+      super.subCaptures(refs1, refs2, frozen)
+    }
+
   def lastTrace(header: String): String = header + { try b.toString finally b.clear() }
 }