String options after settings with colon shouldn't be ignored

Format of some of the compiler settings was incompatible with scalac. For example "-target:jvm-1.8" is a valid argument for scalac, but it should be "-target: jvm-1.8" (with a whitespace) for dotc. This commit make both ways to work.

Author: niktrop

src/dotty/tools/dotc/config/Settings.scala

@@ -115,11 +115,15 @@ object Settings {
         case (ListTag, _) =>
           if (argRest.isEmpty) missingArg
           else update((argRest split ",").toList, args)
-        case (StringTag, arg2 :: args2) =>
-          if (choices.nonEmpty && !(choices contains arg2))
-            fail(s"$arg2 is not a valid choice for $name", args2)
-          else
-            update(arg2, args2)
+        case (StringTag, _) =>
+          if (argRest.isEmpty && args.isEmpty) missingArg
+          else {
+            val (arg, tail) = if (argRest.isEmpty) (args.head, args.tail) else (argRest, args)
+            if (choices.nonEmpty && !(choices contains arg))
+              fail(s"$arg is not a valid choice for $name", tail)
+            else
+              update(arg, tail)
+          }
         case (IntTag, arg2 :: args2) =>
           try {
             val x = arg2.toInt