title: Rcpp implicit construction of DataFrame from S4 slot
body: <p><code>Rcpp::DataFrame pData =pheno.slot("data")</code>  </p>
  
  <p>This fails on clang++ but succeeds with g++</p>
  
  <pre><code>error: conversion from 'Rcpp::SlotProxyPolicy&lt;Rcpp::S4_Impl&lt;PreserveStorage&gt; &gt;::SlotProxy' to 'Rcpp::DataFrame' (aka 'DataFrame_Impl&lt;PreserveStorage&gt;') is ambiguous
              Rcpp::DataFrame fsdata = fspd.slot("data");
                              ^        ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜
  </code></pre>