anchor.diff:  
   [source,c++]
   ----
  -constexpr auto [success, value] = my_lock(key);
  +static_assert( nullopt == my_lock(84) );
  +static_assert( 1337    == my_lock(42) );
   ----
   
   [state=dimbg]
  
positive.code: === ...
  image::art/tree-2649411_1920.jpg[canvas]
  
  [source,c+]
  ----
  constexpr auto make_branch(auto left, auto right)
  {
      return [left,right](int key) -> pair<bool,int>
      {
          if(const auto [success, value] = left(key); success) {
  	    return {true, value};
          }
          return right(key);
      };
  
negative.code: [source,c++]
  ----
  constexpr auto key = 42;
  constexpr auto value = 1337;
  constexpr auto my_lock = make_lock(key, value);
  ----
  
  [state=dimbg]
  