Merge pull request #10052 from griggt/fix-#8927

Fix #8927: Add regression test

Author: liufengyun

tests/pos/i8927.scala

@@ -0,0 +1,23 @@
+import scala.language.implicitConversions
+
+trait Eq[k <: AnyKind, K[_ <: k]]:
+  extension [A <: k, B <: k](k: K[A]) def isEq (k2: K[B]): Eq.GEQ[k, K, A, B]
+
+object Eq:
+  enum GEQ[k <: AnyKind, K[_ <: k], A <: k, B <: k]:
+    case Y[k <: AnyKind, K[_ <: k], A <: k](res: K[A]) extends GEQ[k, K, A, A]
+    case N()
+
+sealed trait DPair[k <: AnyKind, K[_ <: k], +V[_ <: k]]:
+  type A <: k
+  val key: K[A]
+  val value: V[A]
+  final def extract[B <: k](k: K[B])(using Eq[k, K]): Option[V[B]] = key isEq k match
+    case y: Eq.GEQ.Y[k, K, A] => Some(value)
+    case _    => None
+
+object DPair:
+  given pair [k, K[_ <: k], V[_ <: k], C <: k] as Conversion[(K[C], V[C]), DPair[k, K, V]] = tup =>
+    case class dpair(key: K[C], value: V[C]) extends DPair[k, K, V]:
+      type A = C
+    dpair(tup._1, tup._2)