Detect and report double definitions between toplevel files

Detect and report double definitions between members of different toplevel files.
This required a fix in method `precedesIn` for the case where neither owner is
part of the checked baseclass sequence. In that case the answer should be `false`
but we reported `true`.

Author: odersky

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

@@ -412,7 +412,7 @@ object Denotations {
         def precedes(sym1: Symbol, sym2: Symbol) = {
           def precedesIn(bcs: List[ClassSymbol]): Boolean = bcs match {
             case bc :: bcs1 => (sym1 eq bc) || !(sym2 eq bc) && precedesIn(bcs1)
-            case Nil => true
+            case Nil => false
           }
           (sym1 ne sym2) &&
             (sym1.derivesFrom(sym2) ||
@@ -1183,7 +1183,7 @@ object Denotations {
    */
   def isDoubleDef(sym1: Symbol, sym2: Symbol)(implicit ctx: Context): Boolean =
     (sym1.exists && sym2.exists &&
-    (sym1 ne sym2) && (sym1.owner eq sym2.owner) &&
+    (sym1 `ne` sym2) && (sym1.effectiveOwner `eq` sym2.effectiveOwner) &&
     !sym1.is(Bridge) && !sym2.is(Bridge))
 
   def doubleDefError(denot1: Denotation, denot2: Denotation, pre: Type = NoPrefix)(implicit ctx: Context): Nothing = {
@@ -1193,7 +1193,7 @@ object Denotations {
       throw new MergeError(sym1, sym2, sym1.info, sym2.info, pre) {
         override def addendum(implicit ctx: Context) =
           i"""
-             |they are both defined in ${sym1.owner} but have matching signatures
+             |they are both defined in ${sym1.effectiveOwner} but have matching signatures
              |  ${denot1.info} and
              |  ${denot2.info}${super.addendum}"""
         }
@@ -1227,7 +1227,7 @@ object Denotations {
   final case class DenotUnion(denot1: PreDenotation, denot2: PreDenotation) extends MultiPreDenotation {
     def exists: Boolean = true
     def toDenot(pre: Type)(implicit ctx: Context): Denotation =
-      (denot1 toDenot pre) & (denot2 toDenot pre, pre)
+      denot1.toDenot(pre).&(denot2.toDenot(pre), pre)
     def containsSym(sym: Symbol): Boolean =
       (denot1 containsSym sym) || (denot2 containsSym sym)
     type AsSeenFromResult = PreDenotation

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

@@ -169,7 +169,15 @@ class MergeError(val sym1: Symbol, val sym2: Symbol, val tp1: Type, val tp2: Typ
   }
 
   protected def addendum(implicit ctx: Context): String =
-    if (prefix `eq` NoPrefix) "" else i"\nas members of type $prefix"
+    if (prefix `eq` NoPrefix) ""
+    else {
+      val owner = prefix match {
+        case prefix: ThisType => prefix.cls.show
+        case prefix: TermRef => prefix.symbol.show
+        case _ => i"type $prefix"
+      }
+      s"\nas members of $owner"
+    }
 
   override def toMessage(implicit ctx: Context): Message = {
     if (ctx.debug) printStackTrace()

tests/neg/toplevel-doubledef/Test_2.scala

@@ -0,0 +1,3 @@
+object Test extends App{
+  hello("hello") // error: Cannot merge
+}

tests/neg/toplevel-doubledef/defs_1.scala

@@ -0,0 +1,2 @@
+def hello(x: String): Unit =
+  println(s"hello: $x")

tests/neg/toplevel-doubledef/moredefs_1.scala

@@ -0,0 +1,2 @@
+def hello(x: String): Unit =
+  println(s"hi, $x")