title: Why is the trait `core::fmt::Show` not implemented for the type `core::fmt::Show + Sized`?
body: <p>I am trying to compile some code, but I'm getting a pretty weird error:</p>
  
  <blockquote>
    <p>the trait <code>core::fmt::Show</code> is not implemented for the type <code>core::fmt::Show + Sized</code></p>
  </blockquote>
  
  <p>And the code: </p>
  
  <pre><code>use std::fmt::Show;
  
  fn main() {
      println!("{}", return_showed()); // Error occurs here
  }
  
  fn return_showed() -&gt; Box&lt;Show+Sized+'static&gt; {
      box "test" as Box&lt;Show+Sized&gt;
  }
  </code></pre>
  
  <p>This doesn't make much sense to me. Is this a bug in the compiler?</p>
  
  <p>Thanks in advance!</p>