Fix mal-formatting.

Insert an empty line before "where" in an explanation.

Author: odersky

compiler/src/dotty/tools/dotc/core/Decorators.scala

@@ -175,7 +175,7 @@ object Decorators {
      *  give more info about type variables and to disambiguate where needed.
      */
     def ex(args: Any*)(implicit ctx: Context): String =
-      explained2(implicit ctx => em(args: _*))
+      explained(implicit ctx => em(args: _*))
 
     /** Formatter that adds syntax highlighting to all interpolated values */
     def hl(args: Any*)(implicit ctx: Context): String =

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -213,9 +213,11 @@ object Formatting {
     * ex"disambiguate $tpe1 and $tpe2"
     * ```
     */
-  def explained2(op: Context => String)(implicit ctx: Context): String = {
+  def explained(op: Context => String)(implicit ctx: Context): String = {
     val seen = new Seen
-    op(explainCtx(seen)) ++ explanations(seen)
+    val msg = op(explainCtx(seen))
+    val addendum = explanations(seen)
+    if (addendum.isEmpty) msg else msg ++ "\n\n" ++ addendum
   }
 
   /** When getting a type mismatch it is useful to disambiguate placeholders like: