Fix lubbing, push isSoft=false through

Author: dwijnand

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

@@ -2388,7 +2388,7 @@ 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
+    ()