URDF to SDF conversion silently ignores <gazebo> tag with non-existing reference

Desired behavior

If I try to convert the following URDF model, saved in a file called oneLink.urdf:

<?xml version='1.0' encoding='UTF-8'?>
<robot name="oneLink">
    <link name="link1">
        <inertial>
            <mass value="1" />
            <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01" />
        </inertial>
    </link>
    <gazebo reference="lіnk1">
        <sensor name="link1_imu" type="imu">
            <always_on>1</always_on>
            <update_rate>100</update_rate>
            <pose>0.13525 0 -0.07019999999999993 0.0 -0.0 -2.0943952105869315</pose>
            <plugin name="sensor_plugin" filename="example_plugin.so" />
        </sensor>
    </gazebo>
</robot>

to SDF with sdformat 14.0.0 (i.e. via gz sdf -p ./oneLink.urdf > oneLink.sdf), I obtaine the following result, without any additional output:

(gzsdf) traversaro@IITBMP014LW012:~/test34$ gz sdf -p ./oneLink.urdf
<sdf version='1.11'>
  <model name='oneLink'>
    <link name='link1'>
      <inertial>
        <pose>0 0 0 0 0 0</pose>
        <mass>1</mass>
        <inertia>
          <ixx>0.01</ixx>
          <ixy>0</ixy>
          <ixz>0</ixz>
          <iyy>0.01</iyy>
          <iyz>0</iyz>
          <izz>0.01</izz>
        </inertia>
      </inertial>
    </link>
  </model>
</sdf>

Note how the imu sensors is completely ignored, without any warning message or error. Why is this happening? Because, despite the apperance, the link1 string in <link name='link1'> is not the same string as lіnk1 in <gazebo reference="lіnk1">. The second letter of the string in the first case is i , i.e. unicode character U+0069 (Latin Small Letter I), while in the second case is unicode character U+0456 (Cyrillic Small Letter Byelorussian-Ukrainian I).

Clearly I crafted this example to make it explicitly tricky to spot (see https://gist.github.com/StevenACoffman/a5f6f682d94e38ed804182dc2693ed4b if you want to have fun), but in general it would be convenient if the URDF --> SDF parser detected <gazebo reference=".."> that referenced to non-existing elements, and printed at least a warning, to help in case of typos.

Alternatives considered

Continue not to print any warning if a <gazebo> tag refers to a non existing element.

Implementation suggestion

I checked the code, and indeed first the parser parsers all the <gazebo> tags in https://github.com/gazebosim/sdformat/blob/f3607761d3846dba0b660880484ff1e5eb1e9425/src/parser_urdf.cc#L3372, and then only later when it parses links, collisions, visuals and joints, it uses the related sdf extensions if any is found (see for example https://github.com/gazebosim/sdformat/blob/f3607761d3846dba0b660880484ff1e5eb1e9425/src/parser_urdf.cc#L2870C3-L2870C12 or https://github.com/gazebosim/sdformat/blob/f3607761d3846dba0b660880484ff1e5eb1e9425/src/parser_urdf.cc#L3182).

Probably we should add a boolean flag or similar to each SDF extension, that is set to true once a SDF extension is consumed. If an SDF extension is not used at the end of the parsing, a warning should be printed.

Additional context

We experienced this with @Gio-DS .