Close test: GADT constraint drives picking the right implicit (#16807)

Fixes #5469

Author: nicolasstucki

tests/run/i16785.check

@@ -0,0 +1 @@
+specific: 1

tests/run/i16785.scala

@@ -0,0 +1,14 @@
+object Test {
+  sealed trait Box[T] { def value: T }
+  final case class IntBox(value: Int) extends Box[Int]
+
+  implicit def s1[T](implicit box: Box[T]): String = "generic: " + box.value
+  implicit def s2(implicit box: Box[Int]): String = "specific: " + box.value
+
+  def test[T](implicit box: Box[T]): String = box match {
+    case IntBox(_) => implicitly[String]
+  }
+
+  def main(args: Array[String]): Unit =
+    println(test(IntBox(1)))
+}