Add fixed GADT exhaustivity test cases (#16169)

Author: abgruszecki

tests/pos/i7424.scala

@@ -0,0 +1,12 @@
+// scalac: -Werror
+object GADT {
+  import =:=._
+
+  enum =:=[A, B] {
+    case Refl[C]() extends (C =:= C)
+  }
+
+  def unwrap[A,B](opt: Option[A])(using ev: A =:= Option[B]): Option[B] = ev match {
+    case Refl() => opt.flatMap(identity[Option[B]])
+  }
+}

tests/pos/i7424b.scala

@@ -0,0 +1,12 @@
+// scalac: -Werror
+object GADT {
+  import =:=._
+
+  enum =:=[A, B] {
+    case Refl[C]() extends (C =:= C)
+  }
+
+  def unwrap[A,B](opt: Option[A])(using ev: A =:= Option[B]): Option[B] = ev match {
+    case _: Refl[?] => opt.flatMap(identity[Option[B]])
+  }
+}

tests/pos/i7424c.scala

@@ -0,0 +1,8 @@
+// scalac: -Werror
+object Main extends App:
+   enum Extends[A, B]:
+     case Ev[B, A <: B]() extends (A Extends B)
+
+     def cast(a: A): B = this match {
+       case Extends.Ev() => a
+     }