Fix capture set variable installation in Setup

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

@@ -543,7 +543,8 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
     end postProcess
   end setupTraverser
 
-  private def superTypeIsImpure(tp: Type)(using Context): Boolean = {
+  /** Checks whether an abstract type could be impure. See also: [[needsVariable]]. */
+  private def instanceCanBeImpure(tp: Type)(using Context): Boolean = {
     tp.dealiasKeepAnnots match
       case CapturingType(_, refs) =>
         !refs.isAlwaysEmpty
@@ -555,17 +556,18 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           sym == defn.AnyClass
             // we assume Any is a shorthand of {cap} Any, so if Any is an upper
             // bound, the type is taken to be impure.
+          || !sym.isPureClass
         else
-          sym != defn.Caps_Cap && superTypeIsImpure(tp.superType)
+          sym != defn.Caps_Cap && instanceCanBeImpure(tp.superType)
       case tp: (RefinedOrRecType | MatchType) =>
-        superTypeIsImpure(tp.underlying)
+        instanceCanBeImpure(tp.underlying)
       case tp: AndType =>
-        superTypeIsImpure(tp.tp1) || superTypeIsImpure(tp.tp2)
+        instanceCanBeImpure(tp.tp1) || instanceCanBeImpure(tp.tp2)
       case tp: OrType =>
-        superTypeIsImpure(tp.tp1) && superTypeIsImpure(tp.tp2)
+        instanceCanBeImpure(tp.tp1) && instanceCanBeImpure(tp.tp2)
       case _ =>
         false
-  }.showing(i"super type is impure $tp = $result", capt)
+  }.showing(i"instance can be impure $tp = $result", capt)
 
   /** Should a capture set variable be added on type `tp`? */
   def needsVariable(tp: Type)(using Context): Boolean = {
@@ -577,7 +579,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
         else
           val tp1 = tp.dealiasKeepAnnots
           if tp1 ne tp then needsVariable(tp1)
-          else superTypeIsImpure(tp1)
+          else instanceCanBeImpure(tp1)
       case tp: (RefinedOrRecType | MatchType) =>
         needsVariable(tp.underlying)
       case tp: AndType =>
@@ -708,4 +710,4 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
   def postCheck()(using Context): Unit =
     for chk <- todoAtPostCheck do chk(ctx)
     todoAtPostCheck.clear()
-end Setup
+end Setup

tests/pos-custom-args/captures/cc-setup-impure-classes.scala

@@ -0,0 +1,6 @@
+import language.experimental.captureChecking
+
+trait Resource
+def id[X](x: X): x.type = x
+def foo[M <: Resource](r: M^): Unit = id(r)  // was error, should be ok
+def bar[M](r: M^): Unit = id(r)  // ok

tests/pos-custom-args/captures/future-traverse.scala

@@ -0,0 +1,16 @@
+import language.experimental.captureChecking
+
+trait Builder[-A, +C]
+trait BuildFrom[-From, -A, +C] {
+  def newBuilder(from: From): Builder[A, C]
+}
+
+trait Future[+T] { this: Future[T]^ =>
+  import Future.*
+  def foldLeft[R](r: R): R = r
+  def traverse[A, B, M[X] <: IterableOnce[X]](in: M[A]^, bf: BuildFrom[M[A]^, B, M[B]^]): Unit =
+    foldLeft(successful(bf.newBuilder(in)))
+}
+object Future {
+  def successful[T](result: T): Future[T] = ???
+}