Don't check bounds of higher-kinded applications at CC

This is very dubious, but we need it for now to make Tuple go through.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -150,10 +150,10 @@ object Checking {
             // Exempted are types that are not themselves capture-checked.
             // Since the type constructor could not foresee possible capture sets,
             // it's better to be lenient for backwards compatibility.
-            // Also exempted are match aliases. See tuple-ops.scala for an example that
-            // would fail otherwise.
+            // Also exempted are match aliases and higher-kinded applications.
+            // See tuple-ops.scala and tuple-ops-2.scala for examples that would fail otherwise.
             def checkableUnderCC =
-              tycon.typeSymbol.is(CaptureChecked) && !tp.isMatchAlias
+              tycon.typeSymbol.is(CaptureChecked) && !tp.isMatchAlias && !tycon.isLambdaSub
             if !(tycon.typeSymbol.is(JavaDefined) && ctx.compilationUnit.isJava)
                 // Don't check bounds in Java units that refer to Java type constructors.
                 // Scala is not obliged to do Java type checking and in fact i17763 goes wrong

tests/pos-custom-args/captures/tuple-ops-2.scala

@@ -0,0 +1,19 @@
+sealed trait Tup
+case object Emp extends Tup
+type Emp = Emp.type
+case class Cons[h, t <: Tup](hh: h, tt: t) extends Tup
+
+type Union[T <: Tup] = T match
+  case Emp        => Nothing
+  case Cons[h, t] => h | Union[t]
+
+type Concat[T <: Tup, U <: Tup] <: Tup = T match
+  case Emp        => U
+  case Cons[h, t] => Cons[h, Concat[t, U]]
+
+type FlatMap[T <: Tup, F[_ <: Union[T]] <: Tup] <: Tup = T match
+  case Emp        => Emp
+  case Cons[h, t] => Concat[F[h], FlatMap[t, F]]
+
+type A =
+  FlatMap[Cons[Boolean, Cons[String, Emp]], [T] =>> Cons[T, Cons[List[T], Emp]]]