DoubleDefinition: Align description with Scala 2 errors

Author: smarter

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

@@ -2137,7 +2137,7 @@ object messages {
     val explanation: String = ""
   }
 
-  case class DoubleDefinition(decl: Symbol, previousDecl: Symbol)(implicit ctx: Context) extends Message(DoubleDefinitionID) {
+  case class DoubleDefinition(decl: Symbol, previousDecl: Symbol, base: Symbol)(implicit ctx: Context) extends Message(DoubleDefinitionID) {
     val kind: String = "Duplicate Symbol"
     val msg: String = {
       val details = if (decl.isRealMethod && previousDecl.isRealMethod) {
@@ -2157,9 +2157,17 @@ object messages {
             s" at line ${sym.sourcePos.line + 1}" else ""
         i"in ${sym.owner}${lineDesc}"
       }
-      em"""Double definition:
-          |${previousDecl.initial.showDcl} ${symLocation(previousDecl)} and
-          |${decl.initial.showDcl} ${symLocation(decl)}
+      val clashDescription =
+        if (decl.owner eq previousDecl.owner)
+          "Double definition"
+        else if ((decl.owner eq base) || (previousDecl eq base))
+          "Name clash between defined and inherited member"
+        else
+          "Name clash between inherited members"
+
+      em"""$clashDescription:
+          |${previousDecl.showDcl} ${symLocation(previousDecl)} and
+          |${decl.showDcl} ${symLocation(decl)}
           |""" + details
     }
     val explanation: String = ""

compiler/src/dotty/tools/dotc/transform/ElimErasedValueType.scala

@@ -104,7 +104,7 @@ class ElimErasedValueType extends MiniPhase with InfoTransformer {
         //
         // The problem is that `map` was forwarded twice, with different instantiated types.
         // Maybe we should move mixin forwarding after erasure to avoid redundant forwarders like these.
-        ctx.error(DoubleDefinition(sym1, sym2), root.sourcePos)
+        ctx.error(DoubleDefinition(sym1, sym2, root), root.sourcePos)
     }
     val earlyCtx = ctx.withPhase(ctx.elimRepeatedPhase.next)
     while (opc.hasNext) {

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

@@ -755,7 +755,7 @@ trait Checking {
         if (decl.matches(other) && !javaFieldMethodPair) {
           def doubleDefError(decl: Symbol, other: Symbol): Unit =
             if (!decl.info.isErroneous && !other.info.isErroneous)
-              ctx.error(DoubleDefinition(decl, other), decl.sourcePos)
+              ctx.error(DoubleDefinition(decl, other, cls), decl.sourcePos)
           if (decl is Synthetic) doubleDefError(other, decl)
           else doubleDefError(decl, other)
         }

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1514,7 +1514,7 @@ class ErrorMessagesTests extends ErrorMessagesTest {
     }.expect { (ictx, messages) =>
       implicit val ctx: Context = ictx
       assertMessageCount(1, messages)
-      val DoubleDefinition(symbol, previousSymbol) :: Nil = messages
+      val DoubleDefinition(symbol, previousSymbol, _) :: Nil = messages
       assertEquals(symbol.name.mangledString, "a")
   }