Add error message for companion and class/module name clash

Author: rsoeldner

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

@@ -103,6 +103,7 @@ public enum ErrorMessageID {
     ExtendFinalClassID,
     EnumCaseDefinitionInNonEnumOwnerID,
     ExpectedTypeBoundOrEqualsID,
+    ClassAndCompanioNameClashID,
     ;
 
     public int errorNumber() {

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

@@ -1804,4 +1804,14 @@ object messages {
            |refers to a subtype of type ${"A"}.
            |"""
   }
+
+  case class ClassAndCompanionNameClash(cls: Symbol)(implicit ctx: Context)
+    extends Message(ClassAndCompanioNameClashID) {
+    val kind = "Syntax"
+    val msg = hl"Naming clash, ${cls.owner} and it's companion object defines $cls"
+    val explanation =
+      s"""It is not allowed to define a class or module with the same
+        |name inside a class and companion object.
+      """
+  }
 }

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

@@ -115,9 +115,7 @@ object RefChecks {
     if (!(cls.owner is ModuleClass)) {
       val other = cls.owner.linkedClass.info.decl(cls.name)
       if (other.symbol.isClass)
-        ctx.error(s"name clash: ${cls.owner} defines $cls" + "\n" +
-          s"and its companion ${cls.owner.companionModule} also defines $other",
-          cls.pos)
+        ctx.error(ClassAndCompanionNameClash(cls))
     }
 
   // Override checking ------------------------------------------------------------

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

@@ -1038,4 +1038,24 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         val ExpectedTypeBoundOrEquals(found) :: Nil = messages
         assertEquals(Tokens.IDENTIFIER, found)
       }
+
+  @Test def classAndCompanionNameClash =
+    checkMessagesAfter("refchecks") {
+      """
+        |class T {
+        |  class G {}
+        |}
+        |object T {
+        |  class G {}
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(1, messages)
+      val ClassAndCompanionNameClash(symbol) :: Nil = messages
+
+      assertEquals("class T", symbol.owner.show)
+      assertEquals("class G", symbol.show)
+    }
 }