Do seen check only for LazyRef

Also adds test case that stackoverflows without any seen check.

[Cherry-picked 008d58e]

Author: eejbyfeldt

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

@@ -6585,7 +6585,9 @@ object Types extends TypeUtils {
   }
 
   class TypeSizeAccumulator(using Context) extends TypeAccumulator[Int] {
+    var seen = util.HashSet[Type](initialCapacity = 8)
     def apply(n: Int, tp: Type): Int =
+      seen += tp
       tp match {
         case tp: AppliedType =>
           val tpNorm = tp.tryNormalize
@@ -6597,6 +6599,8 @@ object Types extends TypeUtils {
           apply(n, tp.superType)
         case tp: TypeParamRef =>
           apply(n, TypeComparer.bounds(tp))
+        case tp: LazyRef if seen.contains(tp) =>
+          n
         case _ =>
           foldOver(n, tp)
       }

tests/neg/i15692.scala

@@ -0,0 +1,5 @@
+trait TC[X]
+object TC {
+  given [T, S <: TC[S]](using TC[S]): TC[T] = ???
+  summon[TC[Int]] // error
+}

tests/pos/i15692.scala

@@ -9,7 +9,7 @@ class Sum[M <: Nat, N <: Nat] {
 object Sum {
   type Aux[M <: Nat, N <: Nat, R <: Nat] = Sum[M, N] { type Out = R }
 
-  implicit def sum0[N <: Nat]: Sum.Aux[Zero, N, N] = new Sum[Zero, N] { type Out = N } 
+  implicit def sum0[N <: Nat]: Sum.Aux[Zero, N, N] = new Sum[Zero, N] { type Out = N }
   implicit def sum1[M <: Nat, N <: Nat, R <: Nat](implicit sum: Sum.Aux[M, Succ[N], R]): Sum.Aux[Succ[M], N, R] =
     new Sum[Succ[M], N] { type Out = R }
 }