strip nested capturing types before box adaptation

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

@@ -121,6 +121,11 @@ extension (tp: Type)
  case _ =>
  tp
 
+ def isCapturingType(using Context): Boolean =
+ tp match
+ case CapturingType(_, _) => true
+ case _ => false
+
 extension (sym: Symbol)
 
  /** Does this symbol allow results carrying the universal capability?

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

@@ -623,15 +623,19 @@ class CheckCaptures extends Recheck, SymTransformer:
  i"adapting $actual $arrow $expected"
 
  def adapt(actual: Type, expected: Type, covariant: Boolean): Type = trace(adaptInfo(actual, expected, covariant), recheckr, show = true) {
- val actualTp = actual match
- case actual @ CapturingType(parent, cs) =>
- (parent, cs, actual.isBoxed)
+ def destructCapturingType(tp: Type, reconstruct: Type => Type): ((Type, CaptureSet, Boolean), Type => Type) = tp match
+ case tp @ CapturingType(parent, cs) =>
+ if parent.isCapturingType then
+ destructCapturingType(parent, res => reconstruct(tp.derivedCapturingType(res, cs)))
+ else
+ ((parent, cs, tp.isBoxed), reconstruct)
  case actual =>
- (actual, CaptureSet(), false)
+ ((actual, CaptureSet(), false), reconstruct)
 
+ val (actualTp, recon) = destructCapturingType(actual, x => x)
  val (parent1, cs1, isBoxed1) = adaptCapturingType(actualTp, expected, covariant)
 
- CapturingType(parent1, cs1, isBoxed1)
+ recon(CapturingType(parent1, cs1, isBoxed1))
  }
 
  def adaptCapturingType(actual: (Type, CaptureSet, Boolean),
@@ -652,7 +656,8 @@ class CheckCaptures extends Recheck, SymTransformer:
  (aargs1, ares1) =>
  rinfo.derivedLambdaType(paramInfos = aargs1, resType = ares1)
  .toFunctionType(isJava = false, alwaysDependent = true))
- case _ => (parent, cs)
+ case _ =>
+ (parent, cs)
  }
 
  if needsAdaptation then