SI-9369 Fix pattern matcher warnings for diamond shaped inheritance.

A previous optimization (d44a86f432a7f9ca250b014acdeab02ac9f2c304) for
pattern matcher exhaustivity checks used a smarter encoding to ensure
that the scrutinee can be equal to one child only.
However, in case of traits between the root and leave type, a child can
be of several types and these types should not be in a mutually exclusive
group. A simple solution (hat tip to retronym) is to just put traits
and classes into separate groups.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
index a11906ace1..1331eb6993 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
@@ -150,7 +150,11 @@ trait TreeAndTypeAnalysis extends Debugging {
                               acc: List[List[Type]]): List[List[Type]] = wl match {
               case hd :: tl =>
                 val children = enumerateChildren(hd)
-                groupChildren(tl ++ children, acc :+ filterChildren(children))
+                // put each trait in a new group, since traits could belong to the same
+                // group as a derived class
+                val (traits, nonTraits) = children.partition(_.isTrait)
+                val filtered = (traits.map(List(_)) ++ List(nonTraits)).map(filterChildren)
+                groupChildren(tl ++ children, acc ++ filtered)
               case Nil      => acc
             }