New format of illegal start of statement error message (+tests)

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

@@ -2496,8 +2496,7 @@ object Parsers {
           }
         else if (!isStatSep && (in.token != CASE)) {
           exitOnError = mustStartStat
-          val addendum = if (isModifier) " (no modifiers allowed here)" else ""
-          syntaxErrorOrIncomplete("illegal start of statement" + addendum)
+          syntaxErrorOrIncomplete(IllegalStartOfStatement(isModifier))
         }
         acceptStatSepUnlessAtEnd(CASE)
       }

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

@@ -109,7 +109,8 @@ public enum ErrorMessageID {
     OnlyFunctionsCanBeFollowedByUnderscoreID,
     MissingEmptyArgumentListID,
     DuplicateNamedTypeParameterID,
-    UndefinedNamedTypeParameterID
+    UndefinedNamedTypeParameterID,
+    IllegalStartOfStatementID
     ;
 
     public int errorNumber() {

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

@@ -1879,4 +1879,13 @@ object messages {
     val msg = hl"Type parameter $undefinedName is undefined. Expected one of ${definedNames.map(_.show).mkString(", ")}."
     val explanation = ""
   }
+
+  case class IllegalStartOfStatement(isModifier: Boolean)(implicit ctx: Context) extends Message(IllegalStartOfStatementID) {
+    val kind = "Syntax"
+    val msg = {
+      val addendum = if (isModifier) ": no modifiers allowed here" else ""
+      "Illegal start of statement" + addendum
+    }
+    val explanation = "A statement is either an import, a definition or an expression."
+  }
 }

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

@@ -1132,4 +1132,23 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         assertEquals(tpParams, l2.map(_.show))
 
       }
+
+  @Test def illegalStartOfStatement =
+    checkMessagesAfter("frontend") {
+      """
+        |object Test {
+        |  { ) }
+        |  { private ) }
+        |}
+      """.stripMargin
+    }
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+
+        assertMessageCount(2, messages)
+        val errWithModifier :: err :: Nil = messages
+
+        assertEquals(IllegalStartOfStatement(isModifier = false), err)
+        assertEquals(IllegalStartOfStatement(isModifier = true), errWithModifier)
+      }
 }