Add error message - Comments.scala:128

This commit adds the semantic object for the ```definition not found``` error.
It is part of the (https://github.com/lampepfl/dotty/issues/1589)[https://github.com/lampepfl/dotty/issues/1589]

Author: Thiago Pereira

src/dotty/tools/dotc/core/Comments.scala

@@ -9,6 +9,7 @@ import util.Positions._
 import util.CommentParsing._
 import util.Property.Key
 import parsing.Parsers.Parser
+import reporting.diagnostic.messages.ProperDefinitionNotFound
 
 object Comments {
   val ContextDoc = new Key[ContextDocstrings]
@@ -125,7 +126,7 @@ object Comments {
               val newName = (tree.name.show + "$" + codePos + "$doc").toTermName
               untpd.DefDef(newName, tree.tparams, tree.vparamss, tree.tpt, tree.rhs)
             case _ =>
-              ctx.error("proper definition was not found in `@usecase`", codePos)
+              ctx.error(ProperDefinitionNotFound(), codePos)
               tree
           }
         }

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

@@ -512,4 +512,25 @@ object messages {
         |}""".stripMargin
   }
 
+
+  case class ProperDefinitionNotFound()(implicit ctx: Context) extends Message(19) {
+    val kind = "Definition Not Found"
+    val msg = hl"""|Proper definition was not found in ${"@usecase"}"""
+    val explanation = {
+      hl"""|You should use ${"def"} when overriding the method signature on ${"@usecase"}.
+           |
+           |Example:
+           |
+           |${"def map[B, That](f: A => B)(implicit bf: CanBuildFrom[List[A], B, That] : That"}
+           |
+           |Could be written using ${"@usecase"} as follow:
+           |
+           |${"@usecase def map[B](f: A => B): List[B]"}
+           |
+           |This way you can hide the ugly definition by providing an easy readable version.
+           |
+           |""".stripMargin
+    }
+  }
+
 }