Cache widenUnion results

Author: odersky

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

@@ -1282,13 +1282,10 @@ object Types {
       case tp =>
         tp.widenUnionWithoutNull
 
+    /** Overridden in OrType */
     def widenUnionWithoutNull(using Context): Type = widen match
-      case tp @ OrType(lhs, rhs) if tp.isSoft =>
-        TypeComparer.lub(lhs.widenUnionWithoutNull, rhs.widenUnionWithoutNull, canConstrain = true) match
-          case union: OrType => union.join
-          case res => res
-      case tp: AndOrType =>
-        tp.derivedAndOrType(tp.tp1.widenUnionWithoutNull, tp.tp2.widenUnionWithoutNull)
+      case tp: AndType =>
+        tp.derivedAndType(tp.tp1.widenUnionWithoutNull, tp.tp2.widenUnionWithoutNull)
       case tp: RefinedType =>
         tp.derivedRefinedType(tp.parent.widenUnion, tp.refinedName, tp.refinedInfo)
       case tp: RecType =>
@@ -3198,6 +3195,20 @@ object Types {
       myJoin
     }
 
+    private var myUnion: Type = _
+    private var myUnionPeriod: Period = Nowhere
+
+    override def widenUnionWithoutNull(using Context): Type =
+      if myUnionPeriod != ctx.period then
+        myUnion =
+          if isSoft then
+            TypeComparer.lub(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, canConstrain = true) match
+              case union: OrType => union.join
+              case res => res
+          else derivedOrType(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull)
+        if !isProvisional then myUnionPeriod = ctx.period
+      myUnion
+
     private var atomsRunId: RunId = NoRunId
     private var myAtoms: Atoms = _
     private var myWidened: Type = _