Make is type safe

This is achieved by creating a new opaque type `Flag` that represents a single flag.

Author: odersky

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -659,7 +659,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     def isExpanded: Boolean = sym.name.is(ExpandedName)
     def isAnonymousFunction: Boolean = toDenot(sym).isAnonymousFunction
     def isMethod: Boolean = sym.is(Flags.Method)
-    def isPublic: Boolean = !sym.flags.is(Flags.Private | Flags.Protected)
+    def isPublic: Boolean = !sym.flags.isOneOf(Flags.Private | Flags.Protected)
     def isSynthetic: Boolean = sym.is(Flags.Synthetic)
     def isPackageClass: Boolean = sym.is(Flags.PackageClass)
     def isModuleClass: Boolean = sym.is(Flags.ModuleClass)

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1016,9 +1016,9 @@ object desugar {
     case tree: MemberDef =>
       var tested: MemberDef = tree
       def fail(msg: String) = ctx.error(msg, tree.sourcePos)
-      def checkApplicable(flag: FlagSet, test: MemberDefTest): Unit =
+      def checkApplicable(flag: Flag, test: MemberDefTest): Unit =
         if (tested.mods.is(flag) && !test.applyOrElse(tree, (md: MemberDef) => false)) {
-          fail(i"modifier `$flag` is not allowed for this definition")
+          fail(i"modifier `${flag.flagsString}` is not allowed for this definition")
           tested = tested.withMods(tested.mods.withoutFlags(flag))
         }
       checkApplicable(Opaque, legalOpaque)

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -183,8 +183,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     annotations: List[Tree] = Nil,
     mods: List[Mod] = Nil) {
 
-    def is(fs: FlagSet): Boolean = flags.is(fs)
-    def is(fs: FlagSet, butNot: FlagSet): Boolean = flags.is(fs, butNot = butNot)
+    def is(flag: Flag): Boolean = flags.is(flag)
+    def is(flag: Flag, butNot: FlagSet): Boolean = flags.is(flag, butNot = butNot)
     def isOneOf(fs: FlagSet): Boolean = flags.isOneOf(fs)
     def isOneOf(fs: FlagSet, butNot: FlagSet): Boolean = flags.isOneOf(fs, butNot = butNot)
     def isAllOf(fc: FlagSet): Boolean = flags.isAllOf(fc)
@@ -201,8 +201,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       else copy(flags = flags)
 
     def withoutFlags(flags: FlagSet): Modifiers =
-      if (this.is(flags))
-        Modifiers(this.flags &~ flags, this.privateWithin, this.annotations, this.mods.filterNot(_.flags.is(flags)))
+      if (this.isOneOf(flags))
+        Modifiers(this.flags &~ flags, this.privateWithin, this.annotations, this.mods.filterNot(_.flags.isOneOf(flags)))
       else this
 
     def withAddedMod(mod: Mod): Modifiers =