Fast path for ByNameClass in isImpossibleSubtype

These show up often due to the way that searches for
implicit views operate: firstly `A=>B` is sought, and
failing that `(=>A) => B`.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -564,7 +564,7 @@ trait Implicits {
       // side is a class, else we may not know enough.
       case tr1 @ TypeRef(_, sym1, _) if sym1.isClass =>
         tp2.dealiasWiden match {
-          case TypeRef(_, sym2, _)         => sym2.isClass && !(sym1 isWeakSubClass sym2)
+          case TypeRef(_, sym2, _)         => ((sym1 eq ByNameParamClass) != (sym2 eq ByNameParamClass)) || (sym2.isClass && !(sym1 isWeakSubClass sym2))
           case RefinedType(parents, decls) => decls.nonEmpty && tr1.member(decls.head.name) == NoSymbol
           case _                           => false
         }