Propagate hard vars when replacing Constraint params

Author: dwijnand

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
 

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
+}