Make MatchDegree an enum

Author: odersky

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

@@ -757,8 +757,8 @@ object Denotations {
 
     def atSignature(sig: Signature, site: Type, relaxed: Boolean)(implicit ctx: Context): SingleDenotation = {
       val situated = if (site == NoPrefix) this else asSeenFrom(site)
-      val matches = sig.matchDegree(situated.signature) >=
-        (if (relaxed) Signature.ParamMatch else Signature.FullMatch)
+      val matches = sig.matchDegree(situated.signature).ordinal >=
+        (if (relaxed) Signature.ParamMatch else Signature.FullMatch).ordinal
       if (matches) this else NoDenotation
     }
 
@@ -1096,7 +1096,7 @@ object Denotations {
        d == Signature.FullMatch &&
        !infoOrCompleter.isInstanceOf[PolyType] && !other.infoOrCompleter.isInstanceOf[PolyType]
        ||
-       d >= Signature.ParamMatch && info.matches(other.info))
+       d != Signature.NoMatch && info.matches(other.info))
     }
 
     def mapInherited(ownDenots: PreDenotation, prevDenots: PreDenotation, pre: Type)(implicit ctx: Context): SingleDenotation =

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

@@ -107,10 +107,10 @@ case class Signature(paramsSig: List[TypeName], resSig: TypeName) {
 
 object Signature {
 
-  type MatchDegree = Int
-  val NoMatch: Int = 0
-  val ParamMatch: Int = 1
-  val FullMatch: Int = 2
+  enum MatchDegree {
+    case NoMatch, ParamMatch, FullMatch
+  }
+  export MatchDegree._
 
   /** The signature of everything that's not a method, i.e. that has
    *  a type different from PolyType, MethodType, or ExprType.

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

@@ -2154,11 +2154,13 @@ object messages {
       val details = if (decl.isRealMethod && previousDecl.isRealMethod) {
         // compare the signatures when both symbols represent methods
         decl.signature.matchDegree(previousDecl.signature) match {
-          case Signature.NoMatch =>
+          case Signature.MatchDegree.NoMatch =>
+            // DOTTY problem: Need to qualify MatchDegree enum vals since otherwise exhaustivity fails.
+            // To fix this, we need to export vals under singleton types.
             "" // shouldn't be reachable
-          case Signature.ParamMatch =>
+          case Signature.MatchDegree.ParamMatch =>
             "have matching parameter types."
-          case Signature.FullMatch =>
+          case Signature.MatchDegree.FullMatch =>
             i"have the same$nameAnd type after erasure."
         }
       } else ""