Remove seen from TypeSizeAccumulator

This fixes #15692 and does not seem to break an existing compilation
tests. The problem with seen when it comes #15692 is that seen means
that type that has repeated types will get a lower size and this
incorrectly triggers the divergence check since some of the steps
involved less repeated types.

The seen logic was introduced in #6329 and the motivation was to deal
with F-bounds. Since not tests fail it not clear if this logic is still
needed to deal with F-bounds? If it is still needed we can add a test
and instead of removing the seen logic we can make it track only types
the appear as a bound and could cause infinite recursion instead of
tracking all.

[Cherry-picked c2ef180]

Author: Emil Ejbyfeldt

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

@@ -7022,25 +7022,20 @@ 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 =
-      if seen.contains(tp) then n
-      else {
-        seen += tp
-        tp match {
-          case tp: AppliedType =>
-            val tpNorm = tp.tryNormalize
-            if tpNorm.exists then apply(n, tpNorm)
-            else foldOver(n + 1, tp)
-          case tp: RefinedType =>
-            foldOver(n + 1, tp)
-          case tp: TypeRef if tp.info.isTypeAlias =>
-            apply(n, tp.superType)
-          case tp: TypeParamRef =>
-            apply(n, TypeComparer.bounds(tp))
-          case _ =>
-            foldOver(n, tp)
-        }
+      tp match {
+        case tp: AppliedType =>
+          val tpNorm = tp.tryNormalize
+          if tpNorm.exists then apply(n, tpNorm)
+          else foldOver(n + 1, tp)
+        case tp: RefinedType =>
+          foldOver(n + 1, tp)
+        case tp: TypeRef if tp.info.isTypeAlias =>
+          apply(n, tp.superType)
+        case tp: TypeParamRef =>
+          apply(n, TypeComparer.bounds(tp))
+        case _ =>
+          foldOver(n, tp)
       }
   }
 

tests/pos/i15692.scala

@@ -0,0 +1,24 @@
+sealed trait Nat
+sealed trait Succ[Prev <: Nat] extends Nat
+sealed trait Zero extends Nat
+
+class Sum[M <: Nat, N <: Nat] {
+  type Out <: 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 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 }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    type _3 = Succ[Succ[Succ[Zero]]]
+    type _5 = Succ[Succ[_3]]
+
+    implicitly[Sum[_3, _5]]
+  }
+}