Fix #5007 (aka scala/bug#5063)

Based on scala/scala@bed3304bf86 and Nicolas' suggestion.

Author: Blaisorblade

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

@@ -552,6 +552,7 @@ trait Implicits { self: Typer =>
         || (from.tpe isRef defn.NothingClass)
         || (from.tpe isRef defn.NullClass)
         || !(ctx.mode is Mode.ImplicitsEnabled)
+        || from.isInstanceOf[Super]
         || (from.tpe eq NoPrefix)) NoMatchingImplicitsFailure
     else {
       def adjust(to: Type) = to.stripTypeVar.widenExpr match {

tests/neg/t5063.scala

@@ -0,0 +1,3 @@
+class A {
+  super.+("") // error
+}