add gadt exhaustivity check test

Author: liufengyun

tests/patmat/gadt2.scala

@@ -0,0 +1,16 @@
+sealed trait Nat[+T]
+case class Zero() extends Nat[Nothing]
+case class Succ[T]() extends Nat[T]
+
+// +N is incorrect, as in `foo` we can have `N = Zero | Succ[Zero]`,
+// then it's correct for exhaustivity check to produce two warnings.
+sealed trait Vect[N <: Nat[_], +T]
+case class VN[T]() extends Vect[Zero, T]
+case class VC[T, N <: Nat[_]](x: T, xs: Vect[N, T]) extends Vect[Succ[N], T]
+
+object Test {
+  def foo[N <: Nat[_], A, B](v1: Vect[N, A], v2: Vect[N, B]) = (v1, v2) match {
+    case (VN(), VN()) => 1
+    case (VC(x, xs), VC(y, ys)) => 2
+  }
+}

tests/patmat/gadt4.check

@@ -0,0 +1 @@
+12: Pattern Match Exhaustivity: (VC(_, _), VN()), (VN(), VC(_, _))

tests/patmat/gadt2.scala.ignore renamed to tests/patmat/gadt4.scala

@@ -2,6 +2,8 @@ sealed trait Nat[+T]
 case class Zero() extends Nat[Nothing]
 case class Succ[T]() extends Nat[T]
 
+// +N is incorrect, as in `foo` we can have `N = Zero | Succ[Zero]`,
+// then it's correct for exhaustivity check to produce two warnings.
 sealed trait Vect[+N <: Nat[_], +T]
 case class VN[T]() extends Vect[Zero, T]
 case class VC[T, N <: Nat[_]](x: T, xs: Vect[N, T]) extends Vect[Succ[N], T]