Base Modelica class-definition not allowing `class` class-prefix.

[AnHeuermann]

Issue

In the current version of the Base Modelica grammar the class-prefix class is not allowed. How to deal with External Objects from Modelica?
Where should constructor and destructor classes life?
The two constructor and desctructor functions aren't allowed to be called directly (at least in Modelica). How would a Base Modelica parser/ compiler deduce that a function is indeed a constructor/desctructor for an external object and be able to verify that?

Example

When flattening Modelica.Blocks.Examples.ShowLogicalSources (from MSL 4.1) into Base Modelica the external object for the table needs to be lowered to Base Modelica. Currently we at OpenModelica don't know what to do with the ExternalObject class, see OpenModelica/OpenModelica#15238.

The Base Modelica from OpenModelica: Modelica.Blocks.Examples.ShowLogicalSources.bmo

//! base 0.1.0
package 'ShowLogicalSources'
  /* ... */
  class 'Modelica.Blocks.Types.ExternalCombiTimeTable'
  extends ExternalObject;

    function constructor "Initialize 1-dim. table where first column is time"
      input String 'tableName';
      input String 'fileName';
      input Real 'table'[:, :];
      input Real 'startTime';
      input Integer 'columns'[:];
      input 'Modelica.Blocks.Types.Smoothness' 'smoothness';
      input 'Modelica.Blocks.Types.Extrapolation' 'extrapolation';
      input Real 'shiftTime' = 0.0;
      input 'Modelica.Blocks.Types.TimeEvents' 'timeEvents' = 'Modelica.Blocks.Types.TimeEvents'.'Always';
      input Boolean 'verboseRead' = true;
      input String 'delimiter' = ",";
      input Integer 'nHeaderLines' = 0;
      output 'Modelica.Blocks.Types.ExternalCombiTimeTable' 'externalCombiTimeTable';
    external "C" 'externalCombiTimeTable' = ModelicaStandardTables_CombiTimeTable_init3('fileName', 'tableName', 'table', size('table', 1), size('table', 2), 'startTime', 'columns', size('columns', 1), 'smoothness', 'extrapolation', 'shiftTime', 'timeEvents', 'verboseRead', 'delimiter', 'nHeaderLines') annotation(Library = {"ModelicaStandardTables", "ModelicaIO", "ModelicaMatIO", "zlib"}, LibraryDirectory = "modelica://Modelica/Resources/Library");
    end constructor;

    function destructor "Terminate 1-dim. table where first column is time"
      input 'Modelica.Blocks.Types.ExternalCombiTimeTable' 'externalCombiTimeTable';
    external "C" ModelicaStandardTables_CombiTimeTable_close('externalCombiTimeTable') annotation(Library = {"ModelicaStandardTables", "ModelicaIO", "ModelicaMatIO", "zlib"}, LibraryDirectory = "modelica://Modelica/Resources/Library");
    end destructor;

end 'Modelica.Blocks.Types.ExternalCombiTimeTable';

  model 'ShowLogicalSources' "Demonstrates the usage of logical sources together with their diagram animation"
    /* ... */
  parameter 'Modelica.Blocks.Types.ExternalCombiTimeTable' 'table.combiTimeTable.tableID' = 'Modelica.Blocks.Types.ExternalCombiTimeTable'("NoName", "NoName", {{2.0, 0.0}, {2.0, 1.0}, {4.0, 0.0}, {6.0, 1.0}, {8.0, 0.0}}, -1e60, {2}, 'Modelica.Blocks.Types.Smoothness'.'ConstantSegments', 'Modelica.Blocks.Types.Extrapolation'.'HoldLastPoint', 0.0, 'Modelica.Blocks.Types.TimeEvents'.'Always', false, ",", 0) "External table object";
    /* ... */
  end 'ShowLogicalSources';
end 'ShowLogicalSources';

[henrikt-ma]

I think we are generally aware that support for external objects is an open question, and as far as I understand we are waiting for implementations to reach the point where they are ready to propose a solution along with a test implementation. I would therefore reformulate the title of this issue to be about external objects.

If you are at the point where you are ready to propose and discuss possible designs, I think it would be a great idea to do so now. This should avoid the problem of trying to find agreement after the fact when different test implementations have already implemented different designs.