title: When does type information flow backwards in C++?
body: <p>I just watched Stephan T. Lavavej talk at <code>CppCon 2018</code> on "Class Template Argument Deduction", where at <a href="https://youtu.be/-H-ut6j1BYU?t=941" rel="noreferrer">some point</a> he incidentally says:</p>
  
  <blockquote>
    <p>In C++ type information almost never flows backwards ... <strong>I had to say "almost" because there's one or two cases, possibly more but very few</strong>.</p>
  </blockquote>
  
  <p>Despite trying to figure out which cases he might be referring to, I couldn't come up with anything. Hence the question:</p>
  
  <p><strong>In which cases the C++17 standard mandates that type information  propagate backwards?</strong></p>