Formign lubs over widened types only.

To avoid noisy lubs such as Int(1) | Int(2) | Int(3), we impose the rule that A | B is valid only if A and B are non-singleton, widened types. This also maps Nil | Cons automatically to List, and maps None | Option automatically to Some.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index 22d0a1d0b..7b718c9c8 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -558,7 +558,12 @@ class TypeComparer(initctx: Context) extends DotClass {
       else {
         val t2 = mergeIfSuper(tp2, tp1)
         if (t2.exists) t2
-        else orType(tp1, tp2)
+        else {
+          val tp1w = tp1.widen
+          val tp2w = tp2.widen
+          if ((tp1 ne tp1w) && (tp2 ne tp2w)) lub(tp1w, tp2w)
+          else orType(tp1w, tp2w) // no need to check subtypes again
+        }
       }
     }