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.

2 files changed, 25 insertions, 0 deletions

diff --git a/test/files/pos/t9369.flags b/test/files/pos/t9369.flags
new file mode 100644
index 0000000000..b5a8748652
--- /dev/null
+++ b/test/files/pos/t9369.flags
@@ -0,0 +1 @@
+-Xfatal-warnings -unchecked
diff --git a/test/files/pos/t9369.scala b/test/files/pos/t9369.scala
new file mode 100644
index 0000000000..94be2ea4e7
--- /dev/null
+++ b/test/files/pos/t9369.scala
@@ -0,0 +1,24 @@
+object Test {
+
+  trait Tree
+
+  sealed abstract class Prop
+
+  trait Simple extends Prop
+
+  case class Atom(tree: Tree) extends Prop with Simple
+
+  case class Not(prop: Prop) extends Prop with Simple
+
+  def simplify1(prop: Prop): Prop = prop match {
+    case Atom(tree) => ???
+    case Not(prop)  => ???
+    case _          => ???
+  }
+
+  def simplify2(prop: Prop): Prop = prop match {
+    case Not(Atom(tree)) => ???
+    case Not(Not(prop))  => ???
+    case _               => ???
+  }
+}
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