Improve error message

Author: odersky

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -134,7 +134,7 @@ object Flags {
       }
 
     /** The list of non-empty names of flags that are set in the given flag set */
-    def flagStrings(privateWithin: String): Seq[String] = {
+    def flagStrings(privateWithin: String = ""): Seq[String] = {
       var rawStrings = (2 to MaxFlag).flatMap(x.flagString(_)) // DOTTY problem: cannot drop with (_)
       if (!privateWithin.isEmpty && !x.is(Protected))
       	rawStrings = rawStrings :+ "private"

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3847,7 +3847,7 @@ object Parsers {
       def checkLegal(tree: Tree): List[Tree] =
         val problem = tree match
           case tree: MemberDef if !(tree.mods.flags & ModifierFlags).isEmpty =>
-            i"refinement cannot be ${(tree.mods.flags & ModifierFlags).flagsString}"
+            i"refinement cannot be ${(tree.mods.flags & ModifierFlags).flagStrings().mkString("`", "`, `", "`")}"
           case tree: ValOrDefDef =>
             if tree.rhs.isEmpty then ""
             else "refinement in cannot have a right-hand side"

tests/neg/i8150.scala

@@ -1,3 +1,3 @@
 trait A
 trait B
-type T = {given(given a: A) as B}
+type T = {given(given a: A) as B} // error: refinement cannot be `given`