Fix confusing error message

On 2514a.scala, we got

    found   : Int
    expected: Int

It turns out that the real found type was `implicit Int => Int`, so
we should print that.

Author: odersky

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

@@ -104,8 +104,12 @@ object ErrorReporting {
 
     def patternConstrStr(tree: Tree): String = ???
 
-    def typeMismatch(tree: Tree, pt: Type, implicitFailure: SearchFailure = NoImplicitMatches): Tree =
-      errorTree(tree, typeMismatchMsg(normalize(tree.tpe, pt), pt, implicitFailure.postscript))
+    def typeMismatch(tree: Tree, pt: Type, implicitFailure: SearchFailure = NoImplicitMatches): Tree = {
+      val normTp = normalize(tree.tpe, pt)
+      val treeTp = if (normTp <:< pt) tree.tpe else normTp
+        // use normalized type if that also shows an error, original type otherwise
+      errorTree(tree, typeMismatchMsg(treeTp, pt, implicitFailure.postscript))
+    }
 
     /** A subtype log explaining why `found` does not conform to `expected` */
     def whyNoMatchStr(found: Type, expected: Type) = {