Better error message when package names conflict with other defs

Author: odersky

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

@@ -1908,10 +1908,15 @@ object messages {
   }
 
   case class PackageNameAlreadyDefined(pkg: Symbol)(implicit ctx: Context) extends Message(PackageNameAlreadyDefinedID) {
-    val msg: String = em"${pkg} is already defined, cannot be a ${hl("package")}"
+    val (where, or) =
+      if pkg.associatedFile == null then ("", "")
+      else (s" in ${pkg.associatedFile}", " or delete the containing class file")
+    val msg: String = em"""${pkg.name} is the name of $pkg$where.
+                          |It cannot be used at the same time as the name of a package."""
     val kind: String = "Naming"
     val explanation: String =
-      em"An ${hl("object")} cannot have the same name as an existing ${hl("package")}. Rename either one of them."
+      em"""An ${hl("object")} or other toplevel definition cannot have the same name as an existing ${hl("package")}.
+          |Rename either one of them$or."""
   }
 
   case class UnapplyInvalidNumberOfArguments(qual: untpd.Tree, argTypes: List[Type])(implicit ctx: Context)

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

@@ -1879,16 +1879,17 @@ class Typer extends Namer
     val pid1 = typedExpr(tree.pid, AnySelectionProto)(ctx.addMode(Mode.InPackageClauseName))
     val pkg = pid1.symbol
     pid1 match {
-      case pid1: RefTree if pkg.exists =>
-        if (!pkg.is(Package)) ctx.error(PackageNameAlreadyDefined(pkg), tree.sourcePos)
+      case pid1: RefTree if pkg.is(Package) =>
         val packageCtx = ctx.packageContext(tree, pkg)
         var stats1 = typedStats(tree.stats, pkg.moduleClass)(packageCtx)._1
         if (!ctx.isAfterTyper)
           stats1 = stats1 ++ typedBlockStats(MainProxies.mainProxies(stats1))(packageCtx)._1
         cpy.PackageDef(tree)(pid1, stats1).withType(pkg.termRef)
       case _ =>
         // Package will not exist if a duplicate type has already been entered, see `tests/neg/1708.scala`
-        errorTree(tree, i"package ${tree.pid.name} does not exist")
+        errorTree(tree,
+          if pkg.exists then PackageNameAlreadyDefined(pkg)
+          else i"package ${tree.pid.name} does not exist")
     }
   }