Address PR tasks

Author: maseev

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

@@ -1889,20 +1889,20 @@ object messages {
     val explanation = "A statement is either an import, a definition or an expression."
   }
 
-  case class TraitIsExpected(tref: TypeRef)(implicit ctx: Context) extends Message(TraitIsExpectedID) {
+  case class TraitIsExpected(symbol: Symbol)(implicit ctx: Context) extends Message(TraitIsExpectedID) {
     val kind = "Syntax"
-    val msg = hl"$tref is not a trait"
+    val msg = hl"$symbol is not a trait"
     val explanation = {
       val errorCodeExample =
         """class A
           |class B
           |
-          |val a = new A with B // will fail with a compile error - B is not a trait"""
+          |val a = new A with B // will fail with a compile error - class B is not a trait""".stripMargin
       val codeExample =
         """class A
           |trait B
           |
-          |val a = new A with B // compiles normally"""
+          |val a = new A with B // compiles normally""".stripMargin
 
       hl"""Only traits can be mixed into classes using a ${"with"} keyword.
           |Consider the following example:

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

@@ -548,7 +548,7 @@ trait Checking {
   def checkClassType(tp: Type, pos: Position, traitReq: Boolean, stablePrefixReq: Boolean)(implicit ctx: Context): Type =
     tp.underlyingClassRef(refinementOK = false) match {
       case tref: TypeRef =>
-        if (traitReq && !(tref.symbol is Trait)) ctx.error(TraitIsExpected(tref), pos)
+        if (traitReq && !(tref.symbol is Trait)) ctx.error(TraitIsExpected(tref.symbol), pos)
         if (stablePrefixReq && ctx.phase <= ctx.refchecksPhase) checkStable(tref.prefix, pos)
         tp
       case _ =>

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

@@ -1169,7 +1169,7 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       implicit val ctx: Context = ictx
 
       assertMessageCount(1, messages)
-      val TraitIsExpected(tref) :: Nil = messages
-      assertEquals("B", tref.show)
+      val TraitIsExpected(symbol) :: Nil = messages
+      assertEquals("class B", symbol.show)
     }
 }