Fix cutting problem

Test case: orInf.scala. This showed a problem where an `either` operation
had to arbitrarily pick one constraint over another, leading to a type
error down the line. What happened was that a `constrainResult` generated
the constraint

    Set[A] <: Set[String] | Set[Int]

But this constraint cannot be simplified without a cut and a resulting
loss of information. We avoid the problem by not constraining the result
if the prototype is a disjunction.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/typer/ProtoTypes.scala b/src/dotty/tools/dotc/typer/ProtoTypes.scala
index 0e6697fb7..dd5705fbf 100644
--- a/src/dotty/tools/dotc/typer/ProtoTypes.scala
+++ b/src/dotty/tools/dotc/typer/ProtoTypes.scala
@@ -43,6 +43,11 @@ object ProtoTypes {
       isCompatible(normalize(tp, pt)(nestedCtx), pt)(nestedCtx)
     }
 
+    private def disregardProto(pt: Type)(implicit ctx: Context): Boolean = pt.dealias match {
+      case _: OrType => true
+      case pt => pt.isRef(defn.UnitClass)
+    }
+
     /** Check that the result type of the current method
      *  fits the given expected result type.
      */
@@ -54,7 +59,7 @@ object ProtoTypes {
           case _ =>
             true
         }
-      case _: ValueTypeOrProto if !(pt isRef defn.UnitClass) =>
+      case _: ValueTypeOrProto if !disregardProto(pt) =>
         mt match {
           case mt: MethodType =>
             mt.isDependent || isCompatible(normalize(mt, pt), pt)