1 files changed, 61 insertions, 0 deletions
diff --git a/tests/untried/neg-with-implicits/t3346c.scala b/tests/untried/neg-with-implicits/t3346c.scala
new file mode 100644
index 000000000..a5ac166b2
--- /dev/null
+++ b/tests/untried/neg-with-implicits/t3346c.scala
@@ -0,0 +1,61 @@
+object Test extends App {
+  //
+  // An attempt to workaround SI-2712, foiled by SI-3346
+  //
+  trait TC[M[_]]
+
+  type EitherInt[A] = Either[Int, A]
+
+  implicit object EitherTC extends TC[EitherInt]
+
+  def foo[M[_]: TC, A](ma: M[A]) = ()
+
+  val eii: Either[Int, String] = Right("")
+
+  foo[EitherInt, String](eii)
+
+  // This one needs SI-2712 Higher Order Unification
+  //foo(eii) // not inferred
+
+  // A workaround is to provide a set of implicit conversions that take values
+  // based on type constructors of various shapes, and search for the
+  // type class instances.
+  //
+  // This is the approach taken by scalaz7.
+
+  trait TCValue[M[_], A] {
+    implicit def self: M[A]
+    def M: TC[M]
+
+    // instead of `foo(eii)`, we'll try `eii.foo`
+    def foo[M[_], A] = ()
+  }
+
+
+  implicit def ToTCValue[M[_], A](ma: M[A])(implicit M0: TC[M]) = new TCValue[M, A] {
+    implicit val M = M0
+    val self = ma
+  }
+  implicit def ToTCValueBin1[M[_, _], A, B](ma: M[A, B])(implicit M0: TC[({type λ[α]=M[A, α]})#λ]): TCValue[({type λ[α] = M[A, α]})#λ, B] = new TCValue[({type λ[α]=M[A, α]})#λ, B] {
+    implicit val M = M0
+    val self = ma
+  }
+  implicit def ToTCValueBin2[M[_, _], A, B](ma: M[A, B])(implicit M0: TC[({type λ[α]=M[α, B]})#λ]): TCValue[({type λ[α]=M[α, B]})#λ, A] = new TCValue[({type λ[α]=M[α, B]})#λ, A] {
+    implicit val M = M0
+    val self = ma
+  }
+
+
+  ToTCValueBin1(eii).foo
+
+  // as expected, could not find implicit parameter
+  // ToTCValueBin2(eii).bar
+
+  // error: implicit conversions are not applicable because they are ambiguous, both method ToTCValueBin1 ... and  method ToTCValueBin2
+  //        annoying!!
+  //        https://issues.scala-lang.org/browse/SI-3346
+  //
+  // Works if we remove ToTCValueBin2
+  //
+  eii.bar
+}