add testcase for #15921

Author: Linyxus

tests/neg-custom-args/captures/i15921.scala

@@ -0,0 +1,12 @@
+trait Stream { def close(): Unit = (); def write(x: Any): Unit = () }
+
+object Test {
+  def usingLogFile[T](op: (c: {*} Stream) => T): T =
+    val logFile = new Stream { }
+    val result = op(logFile)
+    logFile.close()
+    result
+
+  val later = usingLogFile { f => () => f.write(0) } // error
+  later()   // writing to closed file!
+}