title: Filter dataframe using global variable with the same name as column name
body: <pre><code>library(dplyr)
  </code></pre>
  
  <p>Toy dataset:</p>
  
  <pre><code>df &lt;- data.frame(x = c(1, 2, 3), y = c(4, 5, 6))
  df
    x y
  1 1 4
  2 2 5
  3 3 6
  </code></pre>
  
  <p>This works fine:</p>
  
  <pre><code>df %&gt;% filter(y == 5)
    x y
  1 2 5
  </code></pre>
  
  <p>This also works fine:</p>
  
  <pre><code>z &lt;- 5
  df %&gt;% filter(y == z)
    x y
  1 2 5
  </code></pre>
  
  <p>But this fails</p>
  
  <pre><code>y &lt;- 5
  df %&gt;% filter(y == y)
    x y
  1 1 4
  2 2 5
  3 3 6
  </code></pre>
  
  <p>Apparently, dplyr cannot make the distinction between its column <code>y</code> and the global variable <code>y</code>.
  Is there a way to tell dplyr that the second y is the global variable?</p>