Implement TypeComparer.explaining

Like TypeComparer.explained, but instead of just returning the trace,
returns the result, still allowing the trace to be accessed via
.lastTrace, as exemplified by implementing TypeComparer.explained in
terms of TypeComparer.explaining.

Author: dwijnand

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

@@ -3268,9 +3268,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
   /** The trace of comparison operations when performing `op` */
   def explained[T](op: ExplainingTypeComparer => T, header: String = "Subtype trace:", short: Boolean)(using Context): String =
-    val cmp = explainingTypeComparer(short)
-    inSubComparer(cmp)(op)
-    cmp.lastTrace(header)
+    explaining(cmp => { op(cmp); cmp.lastTrace(header) }, short)
+
+  def explaining[T](op: ExplainingTypeComparer => T, short: Boolean)(using Context): T =
+    inSubComparer(explainingTypeComparer(short))(op)
 
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     inSubComparer(matchReducer)(op)
@@ -3440,6 +3441,9 @@ object TypeComparer {
   def explained[T](op: ExplainingTypeComparer => T, header: String = "Subtype trace:", short: Boolean = false)(using Context): String =
     comparing(_.explained(op, header, short))
 
+  def explaining[T](op: ExplainingTypeComparer => T, short: Boolean = false)(using Context): T =
+    comparing(_.explaining(op, short))
+
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     comparing(_.reduceMatchWith(op))
 
@@ -3873,7 +3877,7 @@ class ExplainingTypeComparer(initctx: Context, short: Boolean) extends TypeCompa
   override def recur(tp1: Type, tp2: Type): Boolean =
     def moreInfo =
       if Config.verboseExplainSubtype || ctx.settings.verbose.value
-      then s" ${tp1.getClass} ${tp2.getClass}"
+      then s" ${tp1.className} ${tp2.className}"
       else ""
     val approx = approxState
     def approxStr = if short then "" else approx.show