Fix #1891: Don't add redundant constraint

Before adding a constraint, make sure there is no way
the two types are already in a subtype relation.

Adding redundant constraints is problematic because we
might introduce cycles. See i1891.scala for a test.

1 files changed, 11 insertions, 0 deletions

diff --git a/tests/pos/i1891.scala b/tests/pos/i1891.scala
new file mode 100644
index 000000000..b178c256b
--- /dev/null
+++ b/tests/pos/i1891.scala
@@ -0,0 +1,11 @@
+object Test {
+  class CC2[A, B](a: A, b: B)
+
+  type T2[A, B] = CC2[A, B]
+
+  class ArrowAssoc[A](val self: A) {
+    @inline def f[B](y: B): CC2[A, B] = new CC2(self, y)
+  }
+
+  def foo = (new ArrowAssoc(1)).f(2)
+}