From ace7c9a2dbe6ed4b80a5ea91afa4414a81472f88 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Zieli=C5=84ski=20Patryk?=
 <75637004+zielinsky@users.noreply.github.com>
Date: Thu, 9 Oct 2025 13:42:07 +0200
Subject: [PATCH] Fix "Regression in zio/zio-schema for typer/implicit
 resolution" (#24156)

Fixes implicit divergence checking regression by correcting how
`TypeSizeAccumulator` handles `TypeParamRef` bounds. Instead of adding
both upper and lower bounds to the type size, we now take the maximum of
both bound sizes.

Fixes https://github.com/scala/scala3/issues/24007
[Cherry-picked 0afa50da6128b04289c108bc54ecf338f3e3e115]
---
 .../src/dotty/tools/dotc/core/Types.scala     |  5 ++-
 tests/pos/i24007.scala                        | 33 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i24007.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 54bfeaadb179..c280d28bcb22 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -6605,7 +6605,10 @@ object Types extends TypeUtils {
         case tp: TypeRef if tp.info.isTypeAlias =>
           apply(n, tp.superType)
         case tp: TypeParamRef =>
-          apply(n, TypeComparer.bounds(tp))
+          val bounds = TypeComparer.bounds(tp)
+          val loSize = apply(n, bounds.lo)
+          val hiSize = apply(n, bounds.hi)
+          hiSize max loSize
         case tp: LazyRef =>
           if seen.contains(tp) then n
           else
diff --git a/tests/pos/i24007.scala b/tests/pos/i24007.scala
new file mode 100644
index 000000000000..86d94211d273
--- /dev/null
+++ b/tests/pos/i24007.scala
@@ -0,0 +1,33 @@
+sealed trait CaseSet:
+  type EnumType
+
+object CaseSet {
+  def caseOf[A, Z >: A]: Cons[A, Empty[Z], Z] = ???
+  type Aux[EnumType0] = CaseSet { type EnumType = EnumType0 }
+
+  final class Empty[Z] extends CaseSet:
+    type EnumType = Z
+
+  final class Cons[A, +T <: CaseSet.Aux[Z], Z](head: A, tail: T) extends CaseSet:
+    type EnumType = Z
+    def ++[That](that: That)(implicit append: Append[Z, Cons[A, T, Z], That]): append.Out = ???
+}
+
+sealed trait Append[EnumType, -Left, -Right]:
+  type Out <: CaseSet.Aux[EnumType]
+
+object Append:
+  type WithOut[EnumType, Left, Right, Out0] = Append[EnumType, Left, Right] { type Out = Out0 }
+
+  implicit def AppendCons[A, Z, T <: CaseSet.Aux[Z], That <: CaseSet.Aux[Z]](implicit
+      append: Append[Z, T, That]
+  ): Append.WithOut[Z, CaseSet.Cons[A, T, Z], That, CaseSet.Cons[A, append.Out, Z]] = ???
+
+  implicit def AppendEmptyLeft[T <: CaseSet.Aux[Z], Z]: Append.WithOut[Z, CaseSet.Empty[Z], T, T] =
+    ???
+
+object Test:
+  type UnionValue = Int | Boolean | String
+  val _ = CaseSet.caseOf[Int, UnionValue] ++
+      CaseSet.caseOf[Boolean, UnionValue] ++
+      CaseSet.caseOf[String, UnionValue]
\ No newline at end of file