Merge pull request #2485 from ennru/ennru_CannotExtendAnyVal

Explanation for 'traits cannot extend AnyVal'

Author: nicolasstucki

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

@@ -1470,7 +1470,11 @@ object messages {
     extends Message(CannotExtendAnyValID) {
     val msg = hl"""$sym cannot extend ${"AnyVal"}"""
     val kind = "Syntax"
-    val explanation = ""
+    val explanation =
+      hl"""Only classes (not traits) are allowed to extend ${"AnyVal"}, but traits may extend
+          |${"Any"} to become ${Green("\"universal traits\"")} which may only have ${"def"} members.
+          |Universal traits can be mixed into classes that extend ${"AnyVal"}.
+          |"""
   }
 
   case class CannotHaveSameNameAs(sym: Symbol, cls: Symbol)(implicit ctx: Context)

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -449,7 +449,7 @@ object Checking {
     }
     if (isDerivedValueClass(clazz)) {
       if (clazz.is(Trait))
-        ctx.error("Only classes (not traits) are allowed to extend AnyVal", clazz.pos)
+        ctx.error(CannotExtendAnyVal(clazz), clazz.pos)
       if (clazz.is(Abstract))
         ctx.error("`abstract' modifier cannot be used with value classes", clazz.pos)
       if (!clazz.isStatic)