Keep softness when widening unions

Author: mbovel

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

@@ -3263,10 +3263,10 @@ object Types {
       if myUnionPeriod != ctx.period then
         myUnion =
           if isSoft then
-            TypeComparer.lub(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, canConstrain = true) match
+            TypeComparer.lub(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, canConstrain = true, isSoft = isSoft) match
               case union: OrType => union.join
               case res => res
-          else derivedOrType(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull)
+          else derivedOrType(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, soft = isSoft)
         if !isProvisional then myUnionPeriod = ctx.period
       myUnion
 
@@ -3282,7 +3282,7 @@ object Types {
           else tp1.atoms | tp2.atoms
         val tp1w = tp1.widenSingletons
         val tp2w = tp2.widenSingletons
-        myWidened = if ((tp1 eq tp1w) && (tp2 eq tp2w)) this else tp1w | tp2w
+        myWidened = if ((tp1 eq tp1w) && (tp2 eq tp2w)) this else TypeComparer.lub(tp1w, tp2w, isSoft = isSoft)
         atomsRunId = ctx.runId
 
     override def atoms(using Context): Atoms =

tests/pos/widen-union.scala

@@ -5,14 +5,19 @@ object Test1:
   val z: Int | String = y
 
 object Test2:
+  val x: 3 | "a" = 3
+  val y = x
+  val z: Int | String = y
+
+object Test3:
   type Sig = Int | String
   def consistent(x: Sig, y: Sig): Boolean = ???// x == y
 
   def consistentLists(xs: List[Sig], ys: List[Sig]): Boolean =
        xs.corresponds(ys)(consistent)        // OK
     || xs.corresponds(ys)(consistent(_, _))  // error, found: Any, required: Int | String
 
-object Test3:
+object Test4:
 
   def g[X](x: X | String): Int = ???
   def y: Boolean | String = ???