title: summarise(n=n()) returns "Evaluation error: This function should not be called directly." even when plyr is not loaded
body: <p>I remove plyr, load dplyr and check the current packages</p>
  
  <pre><code>detach("package:plyr", unload=TRUE)
  library(dplyr)
  (.packages())
  </code></pre>
  
  <blockquote>
    <p>[1] "dplyr"     "bindrcpp"  "stats"     "graphics"  "grDevices"
    "utils"     "datasets"<br>
    [8] "methods"   "base"</p>
  </blockquote>
  
  <p>For info here are the conflicts:</p>
  
  <pre><code>conflicts()
  </code></pre>
  
  <blockquote>
    <p>[1] "filter"    "lag"       "body&lt;-"    "intersect" "kronecker"
    "setdiff"   "setequal"<br>
    [8] "union"</p>
  </blockquote>
  
  <p>Then I use summarise and get the error. This is the same code that I used 6 months ago without issue.</p>
  
  <pre><code>by_vs_am &lt;- group_by(mtcars, vs, am)
  by_vs &lt;- summarise(by_vs_am, n = n())
  </code></pre>
  
  <blockquote>
    <p>Error in summarise_impl(.data, dots) :    Evaluation error: This
    function should not be called directly.</p>
  </blockquote>