From cc55175bd9b4fc04d47448c15d0a68401f01e3d2 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Thu, 12 Oct 2023 11:01:46 +0100
Subject: [PATCH] Propagate hard vars when replacing Constraint params

---
 .../tools/dotc/core/OrderingConstraint.scala  |  3 ++
 tests/pos/i18626.scala                        | 32 +++++++++++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 tests/pos/i18626.scala

diff --git a/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala b/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
index 50e9ac953954..7b94eb3f67d3 100644
--- a/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
+++ b/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
@@ -748,6 +748,9 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
         }
       if isRemovable(param.binder) then current = current.remove(param.binder)
       current.dropDeps(param)
+      replacedTypeVar match
+        case replacedTypeVar: TypeVar if isHard(replacedTypeVar) => current = current.hardenTypeVars(replacement)
+        case _ =>
       current.checkWellFormed()
   end replace
 
diff --git a/tests/pos/i18626.scala b/tests/pos/i18626.scala
new file mode 100644
index 000000000000..0fda265f3812
--- /dev/null
+++ b/tests/pos/i18626.scala
@@ -0,0 +1,32 @@
+trait Random[F1[_]]:
+  def element[T1](list: Seq[T1]): F1[T1] = ???
+
+trait Monad[F2[_]]:
+  def map[A1, B1](fa: F2[A1])(f: A1 => B1): F2[B1]
+
+object Monad:
+  extension [F3[_]: Monad, A3](fa: F3[A3])
+    def map[B3](f: A3 => B3): F3[B3] = ???
+
+sealed trait Animal
+object Cat extends Animal
+object Dog extends Animal
+
+type Mammal = Cat.type | Dog.type
+val mammals: List[Mammal] = ???
+
+class Work[F4[_]](random: Random[F4])(using mf: Monad[F4]):
+  def result1: F4[Mammal] =
+    mf.map(fa = random.element(mammals))(a => a)
+
+  def result2: F4[Mammal] = Monad.map(random.element(mammals))(a => a)
+
+  import Monad.*
+
+  def result3: F4[Mammal] = random
+    .element(mammals)
+    .map { a =>
+       a // was: Type Mismatch Error:
+         //      Found:    (a : Animal)
+         //      Required: Cat.type | Dog.type
+}