Merge pull request #731 from retronym/ticket/5617

SI-5617 Better error message for "x overrides nothing".

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -766,7 +766,16 @@ abstract class RefChecks extends InfoTransform with reflect.internal.transform.R
       for (member <- clazz.info.decls)
         if (member.isAnyOverride && !(clazz.thisType.baseClasses exists (hasMatchingSym(_, member)))) {
           // for (bc <- clazz.info.baseClasses.tail) Console.println("" + bc + " has " + bc.info.decl(member.name) + ":" + bc.info.decl(member.name).tpe);//DEBUG
-          unit.error(member.pos, member.toString() + " overrides nothing");
+
+          val nonMatching: List[Symbol] = clazz.info.member(member.name).alternatives.filterNot(_.owner == clazz).filterNot(_.isFinal)
+          def issueError(suffix: String) = unit.error(member.pos, member.toString() + " overrides nothing" + suffix);
+          nonMatching match {
+            case Nil =>
+              issueError("")
+            case ms =>
+              val superSigs = ms.map(m => m.defStringSeenAs(clazz.tpe memberType m)).mkString("\n")
+              issueError(s".\nNote: the super classes of ${member.owner} contain the following, non final members named ${member.name}:\n${superSigs}")
+          }
           member resetFlag (OVERRIDE | ABSOVERRIDE)  // Any Override
         }
     }

test/files/neg/t5429.check

@@ -46,14 +46,18 @@ t5429.scala:38: error: overriding method emptyArg in class A of type ()Int;
  object emptyArg has incompatible type
   override object emptyArg  // fail
                   ^
-t5429.scala:39: error: object oneArg overrides nothing
+t5429.scala:39: error: object oneArg overrides nothing.
+Note: the super classes of class C contain the following, non final members named oneArg:
+def oneArg(x: String): Int
   override object oneArg    // fail
                   ^
 t5429.scala:43: error: overriding lazy value lazyvalue in class A0 of type Any;
  object lazyvalue must be declared lazy to override a concrete lazy value
   override object lazyvalue // !!! this fails, but should succeed (lazy over lazy)
                   ^
-t5429.scala:46: error: object oneArg overrides nothing
+t5429.scala:46: error: object oneArg overrides nothing.
+Note: the super classes of class C0 contain the following, non final members named oneArg:
+def oneArg(x: String): Any
   override object oneArg    // fail
                   ^
 t5429.scala:50: error: overriding value value in class A of type Int;
@@ -76,7 +80,9 @@ t5429.scala:58: error: overriding lazy value lazyvalue in class A0 of type Any;
  value lazyvalue must be declared lazy to override a concrete lazy value
   override val lazyvalue = 0  // fail (non-lazy)
                ^
-t5429.scala:61: error: value oneArg overrides nothing
+t5429.scala:61: error: value oneArg overrides nothing.
+Note: the super classes of class D0 contain the following, non final members named oneArg:
+def oneArg(x: String): Any
   override val oneArg = 15    // fail
                ^
 t5429.scala:65: error: overriding value value in class A of type Int;
@@ -103,7 +109,9 @@ t5429.scala:73: error: overriding lazy value lazyvalue in class A0 of type Any;
  method lazyvalue needs to be a stable, immutable value
   override def lazyvalue = 2  // fail
                ^
-t5429.scala:76: error: method oneArg overrides nothing
+t5429.scala:76: error: method oneArg overrides nothing.
+Note: the super classes of class E0 contain the following, non final members named oneArg:
+def oneArg(x: String): Any
   override def oneArg = 15    // fail
                ^
 t5429.scala:80: error: overriding value value in class A of type Int;
@@ -126,7 +134,9 @@ t5429.scala:87: error: overriding value value in class A0 of type Any;
  lazy value value cannot override a concrete non-lazy value
   override lazy val value = 0      // fail (strict over lazy)
                     ^
-t5429.scala:91: error: value oneArg overrides nothing
+t5429.scala:91: error: value oneArg overrides nothing.
+Note: the super classes of class F0 contain the following, non final members named oneArg:
+def oneArg(x: String): Any
   override lazy val oneArg = 15    // fail
                     ^
 34 errors found

test/files/neg/t5617.check

@@ -0,0 +1,8 @@
+t5617.scala:12: error: method foo overrides nothing.
+Note: the super classes of trait C contain the following, non final members named foo:
+def foo(u: Unit): Int
+def foo(x: Boolean): Int
+def foo(i: Int)(b: String): Int
+  override def foo(s: String): Int
+               ^
+one error found

test/files/neg/t5617.scala

@@ -0,0 +1,14 @@
+trait A {
+  def foo(i: Int)(b: String): Int
+  def foo(u: Unit): Int // not reported
+  def foo(x: Float): Int // not reported
+}
+trait B[X] extends A {
+  def foo(x: X): Int
+  def foo(u: Unit): Int
+  final def foo(x: Float): Int = 0 // not reported
+}
+trait C extends B[Boolean] {
+  override def foo(s: String): Int
+  val foo = 0 // not reported
+}