Arg files are one arg per line

src/compiler/scala/tools/nsc/CompilerCommand.scala

@@ -126,11 +126,11 @@ class CompilerCommand(arguments: List[String], val settings: Settings) {
   def expandArg(arg: String): List[String] = {
     import java.nio.file.{Files, Paths}
     import scala.jdk.CollectionConverters._
-    def stripComment(s: String) = s.takeWhile(_ != '#')
-    val file = Paths.get(arg stripPrefix "@")
+    def stripComment(s: String) = s.takeWhile(_ != '#').trim()  // arg can be "" but not " "
+    val file = Paths.get(arg.stripPrefix("@"))
     if (!Files.exists(file))
       throw new java.io.FileNotFoundException(s"argument file $file could not be found")
-    settings.splitParams(Files.readAllLines(file).asScala.map(stripComment).mkString(" "))
+    Files.readAllLines(file).asScala.filter(!_.startsWith("#")).map(stripComment).toList
   }
 
   // override this if you don't want arguments processed here

test/files/run/argfile.check

@@ -0,0 +1 @@
+Compiler arguments written to: argfile-run.obj/print-args.txt

test/files/run/argfile.scala

@@ -0,0 +1,39 @@
+
+import java.nio.file.Files
+
+import scala.jdk.CollectionConverters._
+import scala.reflect.internal.util._
+import scala.tools.nsc.{CompilerCommand, Settings}
+import scala.tools.partest.DirectTest
+import scala.util.chaining._
+
+object Test extends DirectTest {
+  var count = 0
+  lazy val argfile = testOutput.jfile.toPath().resolve("print-args.txt")
+  lazy val goodarg = testOutput.jfile.toPath().resolve("print-args2.txt")
+  override def extraSettings =
+    if (count == 0) s"""${super.extraSettings} -Xsource:3 -Vprint-args $argfile "-Wconf:cat=unused-nowarn&msg=does not suppress&site=C:s""""
+    else s"@$goodarg"
+
+  // Use CompilerCommand for expanding the args file.
+  override def newSettings(args: List[String]) = (new Settings).tap { s =>
+    val cc = new CompilerCommand(args, s)
+    assert(cc.ok)
+    assert(cc.files.isEmpty)
+  }
+  def code =
+    sm"""
+    |@annotation.nowarn
+    |final class C {
+    |  def f: Int = "42".toInt
+    |}
+    """
+  def show() = {
+    assert(compile())
+    // drop "-Vprint-args .../print-args.txt newSource1.scala"
+    val args = Files.readAllLines(argfile).asScala.toList.dropRight(3)
+    Files.write(goodarg, args.asJava)
+    count += 1
+    assert(compile())
+  }
+}