Better diagnostic for "failure to disambiguate" errors

Author: odersky

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

@@ -310,12 +310,15 @@ object Denotations {
             }
           case tp1: MethodOrPoly =>
             tp2 match {
-              case tp2: MethodOrPoly
-              if ctx.typeComparer.matchingParams(tp1, tp2) &&
-                 tp1.isImplicit == tp2.isImplicit =>
-                tp1.derivedLambdaType(
-                  mergeParamNames(tp1, tp2), tp1.paramInfos,
-                  infoMeet(tp1.resultType, tp2.resultType.subst(tp2, tp1)))
+              case tp2: MethodOrPoly =>
+                if (tp1.isInstanceOf[PolyType] && tp2.isInstanceOf[MethodType]) tp2
+                else if (tp2.isInstanceOf[PolyType] && tp1.isInstanceOf[MethodType]) tp1
+                else if (ctx.typeComparer.matchingParams(tp1, tp2) &&
+                         tp1.isImplicit == tp2.isImplicit)
+                  tp1.derivedLambdaType(
+                    mergeParamNames(tp1, tp2), tp1.paramInfos,
+                    infoMeet(tp1.resultType, tp2.resultType.subst(tp2, tp1)))
+                else mergeConflict(tp1, tp2)
               case _ =>
                 mergeConflict(tp1, tp2)
             }
@@ -553,7 +556,7 @@ object Denotations {
       if (sd1.exists)
         if (sd2.exists)
           if (isDoubleDef(denot1.symbol, denot2.symbol)) doubleDefError(denot1, denot2)
-          else throw new TypeError(s"failure to disambiguate overloaded reference $this")
+          else throw new TypeError(i"failure to disambiguate overloaded reference at $this")
         else sd1
       else sd2
     }