More explanations for -Ycheck failures.

In case of "types differ" failure, explain why original and retyped type are
not subtypes of each other.

Author: odersky

src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -48,13 +48,19 @@ class TreeChecker {
           def sameType(tp1: Type, tp2: Type) =
             (tp1 eq tp2) || // accept NoType / NoType
             (tp1 =:= tp2)
-          def divergenceMsg =
+          def divergenceMsg = {
+            def explanation(tp1: Type, tp2: Type) =
+              if (tp1 <:< tp2) ""
+              else "\n why different:\n" + core.TypeComparer.explained((tp1 <:< tp2)(_))
             s"""Types differ
                |Original type : ${tree.typeOpt.show}
                |After checking: ${tree1.tpe.show}
                |Original tree : ${tree.show}
                |After checking: ${tree1.show}
-             """.stripMargin
+             """.stripMargin +
+             explanation(tree1.tpe, tree.typeOpt) +
+             explanation(tree.typeOpt, tree1.tpe)
+          }
           assert(sameType(tree1.tpe, tree.typeOpt), divergenceMsg)
           tree1
         }