Add error message IdentifierExpected

This commit adds the semantic object fir the ```identifier expected``` error.
It is part of the #1589

Author: iamthiago

src/dotty/tools/dotc/parsing/JavaParsers.scala

@@ -21,6 +21,7 @@ import Symbols._
 import ast.Trees._
 import Decorators._
 import StdNames._
+import dotty.tools.dotc.reporting.diagnostic.messages.IdentifierExpected
 import dotty.tools.dotc.util.SourceFile
 import util.Positions._
 import annotation.switch
@@ -230,7 +231,7 @@ object JavaParsers {
         case AppliedTypeTree(_, _) | Select(_, _) =>
           tree
         case _ =>
-          syntaxError("identifier expected", tree.pos)
+          syntaxError(IdentifierExpected(), tree.pos)
           errorTypeTree
       }
     }

src/dotty/tools/dotc/parsing/Parsers.scala

@@ -316,7 +316,7 @@ object Parsers {
       case id @ Select(qual, name) =>
         cpy.Select(id)(qual, name.toTypeName)
       case _ =>
-        syntaxError("identifier expected", tree.pos)
+        syntaxError(IdentifierExpected(), tree.pos)
         tree
     }
 

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

@@ -6,11 +6,9 @@ package diagnostic
 import dotc.core._
 import Contexts.Context, Decorators._, Symbols._, Names._, NameOps._, Types._
 import ast.untpd.{Modifiers, ModuleDef}
-import util.{SourceFile, NoSource}
-import util.{SourcePosition, NoSourcePosition}
+import util.SourcePosition
 import config.Settings.Setting
 import interfaces.Diagnostic.{ERROR, WARNING, INFO}
-import dotty.tools.dotc.ast.tpd
 import printing.Highlighting._
 import printing.Formatting
 
@@ -678,4 +676,33 @@ object messages {
            |$fixedVarInAlternative""".stripMargin
     }
   }
+
+  case class IdentifierExpected()(implicit ctx: Context) extends Message(24) {
+    val kind = "Syntax"
+    val msg = "identifier expected"
+
+    val typeExplicitly = "def sum(): Int = {...}"
+
+    val typeImplicit = "def sum() = {...}"
+
+    val explanation = {
+      hl"""|When using type identifiers, you should pass a valid one.
+           |Because of Scala's stronger type, you can fix this issue either
+           |passing a valid type identifier or you can omit it, so the compiler can infer it for you.
+           |
+           |Let's write a function called ${"sum"}
+           |
+           |Here we explicitly pass a valid type identifier, in this case ${"Int"} for the function:
+           |
+           |$typeExplicitly
+           |
+           |But you also have the option to omit it completely.
+           |
+           |$typeImplicit
+           |
+           |This time the compiler will infer this type for you.
+           |
+           |""".stripMargin
+    }
+  }
 }