Rework with explanation error:modifiers(s) not allowed for 'item' (#2747)

* Applies dotty error message standard - modifier(s) 'modifiers' not allowed for 'item'

* restore trailing semicolon

* add trailing ,

Author: johnregan

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

@@ -1681,7 +1681,7 @@ object Parsers {
 
     def addFlag(mods: Modifiers, flag: FlagSet): Modifiers = {
       def incompatible(kind: String) = {
-        syntaxError(s"modifier(s) `${mods.flags}' not allowed for $kind")
+        syntaxError(ModifiersNotAllowed(mods.flags, kind))
         Modifiers(flag)
       }
       if (compatible(mods.flags, flag)) mods | flag

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

@@ -89,7 +89,8 @@ public enum ErrorMessageID {
     OnlyCaseClassOrCaseObjectAllowedID,
     ExpectedClassOrObjectDefID,
     AnonymousFunctionMissingParamTypeID,
-    SuperCallsNotAllowedInlineID
+    SuperCallsNotAllowedInlineID,
+    ModifiersNotAllowedID,
     ;
 
     public int errorNumber() {

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

@@ -20,6 +20,7 @@ import printing.Formatting
 import ErrorMessageID._
 import Denotations.SingleDenotation
 import dotty.tools.dotc.ast.Trees
+import dotty.tools.dotc.ast.untpd.Modifiers
 import dotty.tools.dotc.core.Flags.{FlagSet, Mutable}
 import dotty.tools.dotc.core.SymDenotations.SymDenotation
 
@@ -1614,4 +1615,20 @@ object messages {
     val msg = s"super call not allowed in inline $symbol"
     val explanation = "Method inlining prohibits calling superclass methods, as it may lead to confusion about which super is being called."
   }
+
+  case class ModifiersNotAllowed(flags: FlagSet, sort: String)(implicit ctx: Context)
+    extends Message(ModifiersNotAllowedID) {
+    val kind = "Syntax"
+    val msg = s"modifier(s) `$flags' not allowed for $sort"
+    val explanation = {
+      val code = "sealed def y: Int = 1"
+      hl"""You tried to use a modifier that is inapplicable for the type of item under modification
+         |
+         |
+         |Consider the following example:
+         |$code
+         |In this instance, the modifier 'sealed' is not applicable to the item type 'def' (method)
+        """
+    }
+  }
 }

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

@@ -839,4 +839,14 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         val SuperCallsNotAllowedInline(symbol) = err
         assertEquals("method bar", symbol.show)
       }
+
+  @Test def modifiersNotAllowed =
+    checkMessagesAfter("refchecks")("""lazy trait T""")
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+        assertMessageCount(1, messages)
+        val ModifiersNotAllowed(flags, sort) :: Nil = messages
+        assertEquals("lazy", flags.toString)
+        assertEquals("trait", sort)
+      }
 }