Error on TypedefOrDcl

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

@@ -2156,7 +2156,7 @@ object Parsers {
           case SUPERTYPE | SUBTYPE | SEMI | NEWLINE | NEWLINES | COMMA | RBRACE | EOF =>
             TypeDef(name, lambdaAbstract(tparams, typeBounds())).withMods(mods).setComment(in.getDocComment(start))
           case _ =>
-            syntaxErrorOrIncomplete("`=', `>:', or `<:' expected")
+            syntaxErrorOrIncomplete(ExpectedTypeBoundOrEquals())
             EmptyTree
         }
       }

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

@@ -102,6 +102,7 @@ public enum ErrorMessageID {
     UncheckedTypePatternID,
     ExtendFinalClassID,
     EnumCaseDefinitionInNonEnumOwnerID,
+    ExpectedTypeBoundOrEqualsID,
     ;
 
     public int errorNumber() {

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

@@ -1770,7 +1770,7 @@ object messages {
           |"""
   }
 
-  case class ExtendFinalClass(clazz:Symbol, finalClazz: Symbol)(implicit ctx: Context) 
+  case class ExtendFinalClass(clazz:Symbol, finalClazz: Symbol)(implicit ctx: Context)
     extends Message(ExtendFinalClassID) {
     val kind = "Syntax"
     val msg = hl"$clazz cannot extend ${"final"} $finalClazz"
@@ -1786,4 +1786,23 @@ object messages {
             |If you want to create an ${"enum"} case, make sure the corresponding ${"enum class"} exists
             |and has the ${"enum"} keyword."""
   }
+
+  case class ExpectedTypeBoundOrEquals()(implicit ctx: Context)
+    extends Message(ExpectedTypeBoundOrEqualsID) {
+    val kind = "Syntax"
+    val msg =
+        hl"""`=', `>:', or `<:' expected"""
+
+    val explanation =
+      hl"""|In Scala, type parameters and abstract types may be constrained by a type bound.
+           |Such type bounds limit the concrete values of the type variables and possibly
+           |reveal more information about the members of such types.
+           |
+           |A lower type bound `B >: A` expresses that the type variable `B`
+           |refers to a supertype of type `A`.
+           |
+           |An upper type bound `T <: A` declares that type variable `T`
+           |refers to a subtype of type `A`.
+           |"""
+  }
 }