Do not widen hard unions

Author: mbovel

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

@@ -557,8 +557,8 @@ trait ConstraintHandling {
       // Widening can add extra constraints, in particular the widened type might
       // be a type variable which is now instantiated to `param`, and therefore
       // cannot be used as an instantiation of `param` without creating a loop.
-      // If that happens, we run `instanceType` again to find a new instantation.
-      // (we do not check for non-toplevel occurences: those should never occur
+      // If that happens, we run `instanceType` again to find a new instantiation.
+      // (we do not check for non-toplevel occurrences: those should never occur
       // since `addOneBound` disallows recursive lower bounds).
       if constraint.occursAtToplevel(param, widened) then
         instanceType(param, fromBelow)

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

@@ -3266,7 +3266,7 @@ object Types {
             TypeComparer.lub(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, canConstrain = true) match
               case union: OrType => union.join
               case res => res
-          else derivedOrType(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull)
+          else this
         if !isProvisional then myUnionPeriod = ctx.period
       myUnion
 
@@ -3280,11 +3280,16 @@ object Types {
           if tp1.hasClassSymbol(defn.NothingClass) then tp2.atoms
           else if tp2.hasClassSymbol(defn.NothingClass) then tp1.atoms
           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
         atomsRunId = ctx.runId
 
+        myWidened =
+          if isSoft then
+            val tp1w = tp1.widenSingletons
+            val tp2w = tp2.widenSingletons
+            if ((tp1 eq tp1w) && (tp2 eq tp2w)) this else tp1w | tp2w
+          else
+            this
+
     override def atoms(using Context): Atoms =
       ensureAtomsComputed()
       myAtoms

tests/pos/singleton-widen.scala

@@ -0,0 +1,7 @@
+import scala.compiletime.ops.int._
+
+class Test:
+
+  def test() =
+    val a: 2 | 3 = 2
+    val b = a

tests/pos/widen-union.scala

@@ -13,12 +13,21 @@ object Test2:
     || xs.corresponds(ys)(consistent(_, _))  // error, found: Any, required: Int | String
 
 object Test3:
-
   def g[X](x: X | String): Int = ???
   def y: Boolean | String = ???
   g[Boolean](y)
   g(y)
   g[Boolean](identity(y))
   g(identity(y))
 
+object Test4:
+  def f(a: 2 | 3) = a
+
+  def test() =
+    val x: 2 | 3 = 2
+    val y = x
+    f(y)
 
+    def g: 2 | 3 = 2
+    val z = g
+    f(z)