Make scalac's argument-file processing more like javac's in handling spaces and line breaks

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

@@ -126,11 +126,12 @@ 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(_ != '#').trim()  // arg can be "" but not " "
+    def stripComment(s: String) = s.takeWhile(_ != '#').trim()
     val file = Paths.get(arg.stripPrefix("@"))
     if (!Files.exists(file))
       throw new java.io.FileNotFoundException(s"argument file $file could not be found")
-    Files.readAllLines(file).asScala.filter(!_.startsWith("#")).map(stripComment).toList
+    val lines = Files.readAllLines(file).asScala.map(stripComment).filterNot(_.isEmpty).toList
+    lines.flatMap(settings.splitParams)
   }
 
   // override this if you don't want arguments processed here

src/compiler/scala/tools/nsc/Global.scala

@@ -1492,7 +1492,9 @@ class Global(var currentSettings: Settings, reporter0: Reporter)
 
     private def printArgs(sources: List[SourceFile]): Unit =
       settings.printArgs.valueSetByUser foreach { value =>
-        val argsFile = (settings.recreateArgs ::: sources.map(_.file.absolute.toString())).mkString("", "\n", "\n")
+        def quote(s: String) = if (s.charAt(0) != '"' && s.contains(' ')) "\"" + s + "\"" else s
+        val allArgs = settings.recreateArgs ::: sources.map(_.file.absolute.toString())
+        val argsFile = allArgs.map(quote).mkString("", "\n", "\n")
         value match {
           case "-" =>
             reporter.echo(argsFile)

test/files/run/argfile.scala

@@ -23,7 +23,8 @@ object Test extends DirectTest {
   }
   def code =
     sm"""
-    |@annotation.nowarn
+    |import annotation.*
+    |@nowarn
     |final class C {
     |  def f: Int = "42".toInt
     |}