is is the value type for `file_io_error`. We add line number and source file path.
</span><span class="c1"></span>  <span class="k">struct</span> <span class="n">value_type</span>
  <span class="p">{</span>
    <span class="k">typename</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">posix_code</span><span class="o">::</span><span class="n">value_type</span> <span class="n">errcode</span><span class="p">;</span>  <span class="c1">// from POSIX, as we inherit from _posix_code_d