title: Diagnostic message for well-formed program
body: <p>Is an implementation allowed to issue a diagnostic message for a well-formed program?</p>
  
  <p>For example some compilers issue a warning about unused expression result when compiling the following well-formed program:</p>
  
  <pre><code>int main() { 0; }
  </code></pre>
  
  <p>Are such compilers allowed to consider that warning a diagnostic message?</p>