Fix a recursive TypeBound with some half answers

Author: dwijnand

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

@@ -423,6 +423,8 @@ object TypeOps:
         sym.is(Package) || sym.isStatic && isStaticPrefix(pre.prefix)
       case _ => true
 
+    private var halfAnswers: util.SimpleIdentityMap[Type, Type] = util.SimpleIdentityMap.empty
+
     override def apply(tp: Type): Type = tp match
       case tp: TermRef
       if toAvoid(tp) =>
@@ -434,8 +436,14 @@ object TypeOps:
         tp.info match {
           case info: AliasingBounds =>
             apply(info.alias)
+          case TypeBounds(lo, hi) if halfAnswers.contains(tp) =>
+            halfAnswers(tp)
           case TypeBounds(lo, hi) =>
-            range(atVariance(-variance)(apply(lo)), apply(hi))
+            val hi2 = apply(hi)
+            halfAnswers = halfAnswers.updated(tp, range(defn.NothingType, hi2))
+            val res = range(atVariance(-variance)(apply(lo)), hi2)
+            halfAnswers = halfAnswers.remove(tp)
+            res
           case info: ClassInfo =>
             range(defn.NothingType, apply(classBound(info)))
           case _ =>

tests/pos/i14287.min.scala

@@ -0,0 +1,7 @@
+enum Foo[+H[_]]:
+  case Bar[F[_]](f: Foo[F]) extends Foo[F]
+  case Baz()
+
+  def test: Foo[H] = this match
+    case Bar(Bar(f)) => Bar(f)
+    case _           => this

tests/pos/i14287.scala

@@ -0,0 +1,15 @@
+enum Free[+F[_], A]:
+  case Return(a: A)
+  case Suspend(s: F[A])
+  case FlatMap[F[_], A, B](
+    s: Free[F, A],
+    f: A => Free[F, B]) extends Free[F, B]
+
+  def flatMap[F2[x] >: F[x], B](f: A => Free[F2,B]): Free[F2,B] =
+    FlatMap(this, f)
+
+  @annotation.tailrec
+  final def step: Free[F, A] = this match
+    case FlatMap(FlatMap(fx, f), g) => fx.flatMap(x => f(x).flatMap(y => g(y))).step
+    case FlatMap(Return(x), f) => f(x).step
+    case _ => this