Specialize lub for unions over singletons

Author: odersky

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

@@ -1530,27 +1530,32 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
    *  @note  We do not admit singleton types in or-types as lubs.
    */
   def lub(tp1: Type, tp2: Type, canConstrain: Boolean = false): Type = /*>|>*/ trace(s"lub(${tp1.show}, ${tp2.show}, canConstrain=$canConstrain)", subtyping, show = true) /*<|<*/ {
-    if (tp1 eq tp2) tp1
-    else if (!tp1.exists) tp1
-    else if (!tp2.exists) tp2
-    else if ((tp1 isRef AnyClass) || (tp1 isRef AnyKindClass) || (tp2 isRef NothingClass)) tp1
-    else if ((tp2 isRef AnyClass) || (tp2 isRef AnyKindClass) || (tp1 isRef NothingClass)) tp2
-    else {
-      val t1 = mergeIfSuper(tp1, tp2, canConstrain)
-      if (t1.exists) t1
-      else {
-        val t2 = mergeIfSuper(tp2, tp1, canConstrain)
-        if (t2.exists) t2
-        else {
-          def widen(tp: Type) =
-            if (ctx.scala2Mode || ctx.erasedTypes) tp.widen else tp.widenIfUnstable
-          val tp1w = widen(tp1)
-          val tp2w = widen(tp2)
-          if ((tp1 ne tp1w) || (tp2 ne tp2w)) lub(tp1w, tp2w)
-          else orType(tp1w, tp2w) // no need to check subtypes again
-        }
+    if (tp1 eq tp2) return tp1
+    if (!tp1.exists) return tp1
+    if (!tp2.exists) return tp2
+    if ((tp1 isRef AnyClass) || (tp1 isRef AnyKindClass) || (tp2 isRef NothingClass)) return tp1
+    if ((tp2 isRef AnyClass) || (tp2 isRef AnyKindClass) || (tp1 isRef NothingClass)) return tp2
+    val atoms1 = tp1.atoms
+    if (atoms1.nonEmpty && !ctx.scala2Mode && !ctx.erasedTypes) {
+      val atoms2 = tp2.atoms
+      if (atoms2.nonEmpty) {
+        if (atoms1.subsetOf(atoms2)) return tp2
+        if (atoms2.subsetOf(atoms1)) return tp1
+        if ((atoms1 & atoms2).isEmpty) return orType(tp1, tp2)
       }
     }
+    val t1 = mergeIfSuper(tp1, tp2, canConstrain)
+    if (t1.exists) return t1
+
+    val t2 = mergeIfSuper(tp2, tp1, canConstrain)
+    if (t2.exists) return t2
+
+    def widen(tp: Type) =
+      if (ctx.scala2Mode || ctx.erasedTypes) tp.widen else tp.widenIfUnstable
+    val tp1w = widen(tp1)
+    val tp2w = widen(tp2)
+    if ((tp1 ne tp1w) || (tp2 ne tp2w)) lub(tp1w, tp2w)
+    else orType(tp1w, tp2w) // no need to check subtypes again
   }
 
   /** The least upper bound of a list of types */