closes #3399.

this broke the invariant that argss is a regular matrix

review by odersky

2 files changed, 28 insertions, 0 deletions

diff --git a/test/files/neg/t3399.check b/test/files/neg/t3399.check
new file mode 100644
index 0000000000..eb6c679704
--- /dev/null
+++ b/test/files/neg/t3399.check
@@ -0,0 +1,4 @@
+t3399.scala:23: error: could not find implicit value for parameter e: =:=[Nats.Add[Nats._1,Nats._1],Nats._1]
+    implicitly[ Add[_1, _1] =:= _1]
+              ^
+one error found
diff --git a/test/files/neg/t3399.scala b/test/files/neg/t3399.scala
new file mode 100644
index 0000000000..3edaa0724f
--- /dev/null
+++ b/test/files/neg/t3399.scala
@@ -0,0 +1,24 @@
+object Nats {
+    sealed trait Nat {
+        // fold right on N, N-1, ..., 1
+        type FoldR[Init <: Type, Type, F <: Fold[Nat, Type]] <: Type
+    }
+    sealed trait _0 extends Nat {
+        type FoldR[Init <: Type, Type, F <: Fold[Nat, Type]] = Init
+    }
+    sealed trait Succ[N <: Nat] extends Nat {
+        type FoldR[Init <: Type, Type, F <: Fold[Nat, Type]] =
+          F#Apply[Succ[N], N#FoldR[Init, Type, F]]
+    }
+
+    type Add[A <: Nat, B <: Nat] = A#FoldR[B, Nat, Inc]
+    trait Fold[-Elem, Value] {
+        type Apply[N <: Elem, Acc <: Value] <: Value
+    }
+    type Inc = Fold[Any, Nat] {
+        type Apply[N <: Any, Acc <: Nat] = Succ[Acc]
+    }
+
+    type _1 = Succ[_0]
+    implicitly[ Add[_1, _1] =:= _1]
+}
\ No newline at end of file