Honour hard unions in lubbing and param replacing

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

@@ -138,6 +138,9 @@ abstract class Constraint extends Showable {
   /** The same as this constraint, but with `tv` marked as hard. */
   def withHard(tv: TypeVar)(using Context): This
 
+  /** Mark toplevel type vars in `tp` as hard. */
+  def hardenTypeVars(tp: Type)(using Context): This
+
   /** Gives for each instantiated type var that does not yet have its `inst` field
    *  set, the instance value stored in the constraint. Storing instances in constraints
    *  is done only in a temporary way for contexts that may be retracted

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

@@ -748,9 +748,18 @@ 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
 
+  def hardenTypeVars(tp: Type)(using Context): OrderingConstraint = tp.dealiasKeepRefiningAnnots match
+    case tp: TypeVar if contains(tp.origin) => withHard(tp)
+    case tp: TypeParamRef if contains(tp)   => hardenTypeVars(typeVarOfParam(tp))
+    case tp: AndOrType                      => hardenTypeVars(tp.tp1).hardenTypeVars(tp.tp2)
+    case _                                  => this
+
   def remove(pt: TypeLambda)(using Context): This = {
     def removeFromOrdering(po: ParamOrdering) = {
       def removeFromBoundss(key: TypeLambda, bndss: Array[List[TypeParamRef]]): Array[List[TypeParamRef]] = {

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

@@ -501,17 +501,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             false
         }
 
-        /** Mark toplevel type vars in `tp2` as hard in the current constraint */
-        def hardenTypeVars(tp2: Type): Unit = tp2.dealiasKeepRefiningAnnots match
-          case tvar: TypeVar if constraint.contains(tvar.origin) =>
-            constraint = constraint.withHard(tvar)
-          case tp2: TypeParamRef if constraint.contains(tp2) =>
-            hardenTypeVars(constraint.typeVarOfParam(tp2))
-          case tp2: AndOrType =>
-            hardenTypeVars(tp2.tp1)
-            hardenTypeVars(tp2.tp2)
-          case _ =>
-
         val res = widenOK || joinOK
           || recur(tp11, tp2) && recur(tp12, tp2)
           || containsAnd(tp1)
@@ -534,7 +523,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           // is marked so that it converts all soft unions in its lower bound to hard unions
           // before it is instantiated. The reason is that the variable's instance type will
           // be a supertype of (decomposed and reconstituted) `tp1`.
-          hardenTypeVars(tp2)
+          constraint = constraint.hardenTypeVars(tp2)
 
         res
 
@@ -2388,7 +2377,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
               case Atoms.Range(lo2, hi2) =>
                 if hi1.subsetOf(lo2) then return tp2
                 if hi2.subsetOf(lo1) then return tp1
-                if (hi1 & hi2).isEmpty then return orType(tp1, tp2)
+                if (hi1 & hi2).isEmpty then return orType(tp1, tp2, isSoft = isSoft)
               case none =>
           case none =>
         val t1 = mergeIfSuper(tp1, tp2, canConstrain)

tests/pos/i18626.min1.scala

@@ -0,0 +1,14 @@
+sealed trait Animal
+object Cat extends Animal
+object Dog extends Animal
+
+type Mammal = Cat.type | Dog.type
+
+class Test:
+  def t1 =
+    val mammals: List[Mammal] = ???
+    val result = mammals.head
+    val mammal: Mammal = result // was: Type Mismatch Error:
+                                //      Found:    (result : Animal)
+                                //      Required: Mammal
+    ()

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
+}