Test interaction between opaque and match

Author: Aleksander Boruch-Gruszecki

tests/pos/opaques-patmat.scala

@@ -0,0 +1,25 @@
+object OpaquePatmat {
+  opaque type Pos = Int
+  object Pos {
+    def mkPos(i: Int): Pos = {
+      require(i > 0)
+      i
+    }
+    def coerce[F[_]](fa: F[Int]): F[Pos] = fa
+  }
+  
+  sealed trait Expr[T]
+  final case class PosExpr(p: Pos) extends Expr[Pos]
+  final case class IntExpr(i: Int) extends Expr[Int]
+  final case class StrExpr(s: String) extends Expr[String]
+  
+  def eval(e: Expr[Pos]): Pos = e match {
+    case PosExpr(p) => p
+    // both of the patterns need to be well-typed here,
+    // since Pos is potentially equal to any other type
+    case IntExpr(_) => Pos.mkPos(1)
+    case StrExpr(_) => ???
+  }
+  
+  eval(Pos.coerce[Expr](IntExpr(-1)))
+}