Refactor type parameter widening

Author: Linyxus

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

@@ -1310,12 +1310,14 @@ class CheckCaptures extends Recheck, SymTransformer:
  testAdapted(actual, expected, tree, addenda)(err.typeMismatch)
 
  @annotation.tailrec
- private def widenNamed(tp: Type)(using Context): Type = tp match
- case stp: SingletonType => widenNamed(stp.widen)
- case ntp: NamedType => ntp.info match
- case info: TypeBounds => widenNamed(info.hi)
- case _ => tp
- case _ => tp
+ private def findImpureUpperBound(tp: Type)(using Context): Type = tp match
+ case _: SingletonType => findImpureUpperBound(tp.widen)
+ case tp: TypeRef if tp.symbol.isAbstractOrParamType =>
+ tp.info match
+ case TypeBounds(_, hi) if hi.isBoxedCapturing => hi
+ case TypeBounds(_, hi) => findImpureUpperBound(hi)
+ case _ => NoType
+ case _ => NoType
 
  inline def testAdapted(actual: Type, expected: Type, tree: Tree, addenda: Addenda)
  (fail: (Tree, Type, Addenda) => Unit)(using Context): Type =
@@ -1332,7 +1334,8 @@ class CheckCaptures extends Recheck, SymTransformer:
  //
  // Therefore, when the expected type is a selection proto, we conservatively widen
  // the actual type to strip type parameters.
- widenNamed(actual)
+ val hi = findImpureUpperBound(actual)
+ if !hi.exists then actual else hi
  else actual
  val actualBoxed = adapt(actual1, expected1, tree)
  //println(i"check conforms $actualBoxed <<< $expected1")
@@ -1357,8 +1360,8 @@ class CheckCaptures extends Recheck, SymTransformer:
  * In those cases, we widen such types and try box adaptation another time.
  */
  def tryWidenNamed: Boolean =
- val actual1 = widenNamed(actual)
- (actual1 ne actual) && {
+ val actual1 = findImpureUpperBound(actual)
+ actual1.exists && {
  val actualBoxed1 = adapt(actual1, expected1, tree)
  isCompatible(actualBoxed1, expected1)
  }