Add test showing injectivity issue

Author: Aleksander Boruch-Gruszecki

tests/gadt-neg/uninjectivity.scala

@@ -0,0 +1,24 @@
+object uninjectivity {
+  sealed trait EQ[A, B]
+  final case class Refl[T]() extends EQ[T, T]
+
+  def absurd1[F[_], X, Y](eq: EQ[F[X], F[Y]], x: X): Y = eq match {
+    case Refl() =>
+      x // should be an error
+  }
+
+  def absurd2[F[_], G[_]](eq: EQ[F[Int], G[Int]], fi: F[Int], fs: F[String]): G[Int] = eq match {
+    case Refl() =>
+      val gs: G[String] = fs // error
+      // fi
+      ???
+  }
+
+  def absurd3[F[_], G[_], X, Y](eq: EQ[F[X], G[Y]], fx: F[X]): G[Y] = eq match {
+    case Refl() =>
+      val gx: G[X] = fx // error
+      val fy: F[Y] = fx // error
+      // fx
+      ???
+  }
+}