title: lubridate masking dplyr "union" but no mask message when loaded
body: <p>I'm learning R and wondering whether lubridate should have issued a message about masking "union" from dplyr.  </p>
  
  <p>With dplyr loaded before lubridate, I get an error on the :arrange":</p>
  
  <pre><code>library(dplyr)
  Attaching package: ‘dplyr’
  
  The following objects are masked from ‘package:stats’:
  
  filter, lag
  
  The following objects are masked from ‘package:base’:
  
  intersect, setdiff, setequal, union
  
  library(lubridate)
  df1 &lt;- data.frame(c1 = "a", c2 = 1)
  df2 &lt;- data.frame(c1 = "a", c2 = 2)
  union(df1, df2) %&gt;% arrange(c1)
  Error in UseMethod("arrange_") : 
    no applicable method for 'arrange_' applied to an object of class "list"
  </code></pre>
  
  <p>Seems that the union returns a list instead of a data.frame, and the arrange then trips on it:</p>
  
  <pre><code>str(union(df1, df2))
  List of 3
   $ : Factor w/ 1 level "a": 1
   $ : num 1
   $ : num 2
  </code></pre>
  
  <p>I eventually determined that lubridate has a "union" function which is what was apparently being called instead of the dplyr "union". Specifically asking for dplyr "union" does the trick:</p>
  
  <pre><code>str(dplyr::union(df1, df2))
  'data.frame':   2 obs. of  2 variables:
   $ c1: Factor w/ 1 level "a": 1 1
   $ c2: num  1 2
  
  dplyr::union(df1, df2) %&gt;% arrange(c1)
    c1 c2
  1  a  1
  2  a  2
  </code></pre>
  
  <p>Or, if lubridate is loaded first, all is well. Here's an example (after a restart of RStudio): </p>
  
  <pre><code>library(lubridate)
  library(dplyr)
  
  Attaching package: ‘dplyr’
  
  The following objects are masked from ‘package:lubridate’:
  
  intersect, setdiff, union
  
  The following objects are masked from ‘package:stats’:
  
  filter, lag
  
  The following objects are masked from ‘package:base’:
  
  intersect, setdiff, setequal, union
  
  df1 &lt;- data.frame(c1 = "a", c2 = 1)
  df2 &lt;- data.frame(c1 = "a", c2 = 2)
  union(df1, df2) %&gt;% arrange(c1)
    c1 c2
  1  a  1
  2  a  2 
  </code></pre>
  
  <p>The clue was seeing that dplyr masked "union" from lubridate (although, the first clue was the "no applicable method" message, but I didn't know yet what was going on - I will next time). I would have expected, however, that when lubridate is loaded after dplyr, a similar message would have been issued to indicate that lubridate would be masking "union" from dplyr. Is there a reason why such a message did not appear? Maybe something I need to enable in my setup?</p>
  
  <p>I'm using RStudio version 0.99.489, R version 3.2.3, dplyr version 0.4.3, and lubridate version 1.5.0 on Windows 7.</p>