Merge pull request scala#7012 from milessabin/topic/faster-cores

Speed up implicit resolution by avoiding allocations when traversing TypeRefs in core

Author: adriaanm

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

@@ -482,7 +482,10 @@ trait Implicits {
       case AnnotatedType(annots, tp)          => core(tp)
       case ExistentialType(tparams, result)   => core(result).subst(tparams, tparams map (t => core(t.info.bounds.hi)))
       case PolyType(tparams, result)          => core(result).subst(tparams, tparams map (t => core(t.info.bounds.hi)))
-      case TypeRef(pre, sym, args)            => typeRef(pre, sym, args.map(core))
+      case TypeRef(pre, sym, args)            =>
+        val coreArgs = args.mapConserve(core)
+        if (coreArgs eq args) tp
+        else typeRef(pre, sym, coreArgs)
       case _                                  => tp
     }