Better error message for RefChecks

Trying to diagnose failure of t2503 and t5577 which were moved
to disabled in the time the tests did not work. These failed
locally for me, but the checkin tests succeeded (since the checkin
tests tested something else).

Added better diagnostics to RefChecks which now mention candidate
implementations of missing abstract members that have the right name but
not the right type.

Author: odersky

src/dotty/tools/dotc/typer/RefChecks.scala

@@ -420,7 +420,7 @@ object RefChecks {
         for (member <- missing) {
           val memberSym = member.symbol
           def undefined(msg: String) =
-            abstractClassError(false, member.showDcl + " is not defined" + msg)
+            abstractClassError(false, s"${member.showDcl} is not defined $msg")
           val underlying = memberSym.underlyingSymbol
 
           // Give a specific error message for abstract vars based on why it fails:
@@ -456,9 +456,8 @@ object RefChecks {
                   case (pa, pc) :: Nil =>
                     val abstractSym = pa.typeSymbol
                     val concreteSym = pc.typeSymbol
-                    def subclassMsg(c1: Symbol, c2: Symbol) = (
-                      ": %s is a subclass of %s, but method parameter types must match exactly.".format(
-                        c1.showLocated, c2.showLocated))
+                    def subclassMsg(c1: Symbol, c2: Symbol) =
+                      s": ${c1.showLocated} is a subclass of ${c2.showLocated}, but method parameter types must match exactly."
                     val addendum =
                       if (abstractSym == concreteSym) {
                         val paArgs = pa.argInfos
@@ -478,12 +477,14 @@ object RefChecks {
                         subclassMsg(concreteSym, abstractSym)
                       else ""
 
-                    undefined("\n(Note that %s does not match %s%s)".format(pa, pc, addendum))
+                    undefined(s"\n(Note that $pa does not match $pc$addendum)")
                   case xs =>
-                    undefined("")
+                    undefined(s"\n(The class implements a member with a different type: ${concrete.showDcl})")
                 }
-              case _ =>
+              case Nil =>
                 undefined("")
+              case concretes =>
+                undefined(s"\n(The class implements members with different types: ${concretes.map(_.showDcl)}%\n  %)")
             }
           } else undefined("")
         }