new format for package name already defined error (+tests)

Author: Aurélien Richez

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -109,7 +109,8 @@ public enum ErrorMessageID {
     OnlyFunctionsCanBeFollowedByUnderscoreID,
     MissingEmptyArgumentListID,
     DuplicateNamedTypeParameterID,
-    UndefinedNamedTypeParameterID
+    UndefinedNamedTypeParameterID,
+    PackageNameAlreadyDefinedID
     ;
 
     public int errorNumber() {

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

@@ -1879,4 +1879,12 @@ object messages {
     val msg = hl"Type parameter $undefinedName is undefined. Expected one of ${definedNames.map(_.show).mkString(", ")}."
     val explanation = ""
   }
+
+  case class PackageNameAlreadyDefined(pkg: Symbol)(implicit ctx: Context) extends Message(PackageNameAlreadyDefinedID) {
+
+    override def msg: String = hl"${pkg} is already defined, cannot be a package"
+    override def kind: String = "Syntax"
+    override def explanation: String =
+      "An object cannot have the same name as an existing package. Rename ${pkg} or the package with the same name."
+  }
 }

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

@@ -1516,7 +1516,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       val packageContext =
         if (pkg is Package) ctx.fresh.setOwner(pkg.moduleClass).setTree(tree)
         else {
-          ctx.error(em"$pkg is already defined, cannot be a package", tree.pos)
+          ctx.error(PackageNameAlreadyDefined(pkg), tree.pos)
           ctx
         }
       val stats1 = typedStats(tree.stats, pkg.moduleClass)(packageContext)

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

@@ -1,4 +1,5 @@
 package dotty.tools
+
 package dotc
 package reporting
 
@@ -1132,4 +1133,18 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         assertEquals(tpParams, l2.map(_.show))
 
       }
+
+  @Test def packageNameAlreadyDefined =
+    checkMessagesAfter("frontend") {
+      """
+        |package bar { }
+        |object bar { }
+        |
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(1, messages)
+      assert(messages.head.isInstanceOf[PackageNameAlreadyDefined])
+    }
 }