Make Formatting#explained take an implicit function type.

Author: odersky

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

@@ -204,7 +204,7 @@ object Decorators {
      *  give more info about type variables and to disambiguate where needed.
      */
     def ex(args: Any*)(implicit ctx: Context): String =
-      explained(implicit ctx => em(args: _*))
+      explained(em(args: _*))
   }
 
   implicit class ArrayInterpolator[T <: AnyRef](val arr: Array[T]) extends AnyVal {

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

@@ -252,9 +252,9 @@ object Formatting {
     * ex"disambiguate $tpe1 and $tpe2"
     * ```
     */
-  def explained(op: Context => String)(implicit ctx: Context): String = {
+  def explained(op: given Context => String)(implicit ctx: Context): String = {
     val seen = new Seen
-    val msg = op(explainCtx(seen))
+    val msg = op given explainCtx(seen)
     val addendum = explanations(seen)
     if (addendum.isEmpty) msg else msg ++ "\n\n" ++ addendum
   }