Add some missing calls to flagString

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -464,9 +464,9 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     def isModuleClassRoot = nameMatches(moduleClassRoot.name) && (owner == moduleClassRoot.owner) && flags.is(Module)
     def isModuleRoot = nameMatches(moduleClassRoot.name.sourceModuleName) && (owner == moduleClassRoot.owner) && flags.is(Module)
 
-    //if (isClassRoot) println(s"classRoot of $classRoot found at $readIndex, flags = $flags") // !!! DEBUG
-    //if (isModuleRoot) println(s"moduleRoot of $moduleRoot found at $readIndex, flags = $flags") // !!! DEBUG
-    //if (isModuleClassRoot) println(s"moduleClassRoot of $moduleClassRoot found at $readIndex, flags = $flags") // !!! DEBUG
+    //if (isClassRoot) println(s"classRoot of $classRoot found at $readIndex, flags = ${flags.flagsString}") // !!! DEBUG
+    //if (isModuleRoot) println(s"moduleRoot of $moduleRoot found at $readIndex, flags = ${flags.flagsString}") // !!! DEBUG
+    //if (isModuleClassRoot) println(s"moduleClassRoot of $moduleClassRoot found at $readIndex, flags = ${flags.flagsString}") // !!! DEBUG
 
     def completeRoot(denot: ClassDenotation, completer: LazyType): Symbol = {
       denot.setFlag(flags)

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

@@ -1504,7 +1504,7 @@ object messages {
   case class AbstractMemberMayNotHaveModifier(sym: Symbol, flag: FlagSet)(
     implicit ctx: Context)
     extends Message(AbstractMemberMayNotHaveModifierID) {
-    val msg: String = em"""${hl("abstract")} $sym may not have `$flag' modifier"""
+    val msg: String = em"""${hl("abstract")} $sym may not have `${flag.flagsString}' modifier"""
     val kind: String = "Syntax"
     val explanation: String = ""
   }

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -394,10 +394,10 @@ object Checking {
     def checkNoConflict(flag1: FlagSet, flag2: FlagSet, msg: => String) =
       if (sym.isAllOf(flag1 | flag2)) fail(msg)
     def checkCombination(flag1: FlagSet, flag2: FlagSet) =
-      checkNoConflict(flag1, flag2, i"illegal combination of modifiers: `$flag1` and `$flag2` for: $sym")
+      checkNoConflict(flag1, flag2, i"illegal combination of modifiers: `${flag1.flagsString}` and `${flag2.flagsString}` for: $sym")
     def checkApplicable(flag: FlagSet, ok: Boolean) =
       if (!ok && !sym.is(Synthetic))
-        fail(i"modifier `$flag` is not allowed for this definition")
+        fail(i"modifier `${flag.flagsString}` is not allowed for this definition")
 
     if (sym.is(Inline) &&
           (  sym.is(ParamAccessor) && sym.owner.isClass