Regression test for #16224 (#18067)

Fixes #16224

Author: odersky

tests/pos-custom-args/captures/i16224.scala

@@ -0,0 +1,14 @@
+import language.experimental.captureChecking
+class Delta:
+  val value = 1
+
+  def f(v: Int)(using delta: Delta): Int =
+    v + delta.value
+
+  def run(): Unit =
+    val delta = Delta()
+    val x: Map[Char, Int] = Map(
+      'a' -> 0,
+      'b' -> 1
+    )
+    val y: Map[Char, Int] = x.map((k, v) => (k, f(v)(using delta)))