Fix #3614: Do not allow the SyntaxFormatter to recolor code

Author: nicolasstucki

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -77,17 +77,16 @@ object Formatting {
   class SyntaxFormatter(sc: StringContext) extends StringFormatter(sc) {
     override protected def showArg(arg: Any)(implicit ctx: Context): String =
       arg match {
-        case arg: Showable if ctx.settings.color.value != "never" =>
-          val highlighted =
-            SyntaxHighlighting(wrapNonSensical(arg, super.showArg(arg)))
-          new String(highlighted.toArray)
         case hl: Highlight =>
           hl.show
         case hb: HighlightBuffer =>
           hb.toString
         case str: String if ctx.settings.color.value != "never" =>
           new String(SyntaxHighlighting(str).toArray)
-        case _ => super.showArg(arg)
+        case x :: xs =>
+          showArg(x) + (if (xs.isEmpty) "" else ", " + showArg(xs))
+        case _ =>
+          super.showArg(arg)
       }
   }
 

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -714,8 +714,6 @@ object messages {
       .map(_.paramName.unexpandedName.show)
       .mkString("[", ", ", "]")
 
-    private val actualArgString = actual.map(_.show).mkString("[", ", ", "]")
-
     private val prettyName =
       try
         fntpe.termSymbol match {
@@ -729,7 +727,7 @@ object messages {
     val msg =
       hl"""|${NoColor(msgPrefix)} type arguments for $prettyName$expectedArgString
            |expected: $expectedArgString
-           |actual:   $actualArgString""".stripMargin
+           |actual:   [$actual]""".stripMargin
 
     val explanation = {
       val tooManyTypeParams =