title: Counterpart to std::enable_if_t
body: <p><code>std::enable_if_t</code> gets you a type if a certain value is true. Now I want a thing that gets you a type if an expression is well-formed. If the type I want is <code>void</code>, I can do <code>decltype((void)(expr))</code>. But what should I do if I want something other than <code>void</code>? I want an succinct and elegant solution.</p>