\cs_new_protected:Nn  % warning on this line
  \example_expandable_first:
  { foo }
\cs_new_protected:Nn  % warning on this line
  \example_expandable_second:
  { \prg_do_nothing: }
\cs_new:Nn
  \example_expandable_third:
  { bar }
\cs_new_protected:Nn  % warning on this line
  \example_expandable_fourth:
  { \example_expandable_third: }