add fallback if indexed objects are missing

tagpdf-backend.dtx

@@ -706,7 +706,13 @@ end
 % The return value is |n 0 R|, if the object doesn't exist, n is 0.
 %    \begin{macrocode}
 local function @@_pdf_object_ref (name,index)
-   local object = ltx.pdf.object_id (name,index) ..' 0 R'
+   local object
+   if ltx.pdf.object_id then
+     object = ltx.pdf.object_id (name,index) ..' 0 R'
+   else
+     local tokenname = 'c__pdf_object_'..name..'/'..index..'_int'
+     object = token.create(tokenname).mode ..' 0 R'
+   end 
    return object
 end
 ltx.@@.func.pdf_object_ref = @@_pdf_object_ref

tagpdf.dtx

@@ -184,7 +184,34 @@
  }
 %</base>
 %    \end{macrocode}
+% \subsection{Indexed objects}
+% 2024-04-11: Temporary code! Can be removed after the next expl3 release!
+%    \begin{macrocode}
+\cs_if_free:NT \pdf_object_new_indexed:nn
+ {
+   \cs_generate_variant:Nn \pdf_object_new:n {e}
+   \cs_generate_variant:Nn \pdf_object_write:nnn {enn}
+   \cs_new_protected:Npn \pdf_object_new_indexed:nn #1 #2 
+     {
+       \pdf_object_new:e {#1/\int_eval:n{#2}}
+     }
+   \cs_new_protected:Npn \pdf_object_write_indexed:nnnn #1 #2 #3 #4
+     {
+       \pdf_object_write:enn {#1/\int_eval:n{#2}}{#3}{#4}
+     }   
+   \cs_generate_variant:Nn \pdf_object_write_indexed:nnnn {nnne}
+   \cs_new:Npn\pdf_object_ref_indexed:nn #1 #2
+     {
+       \pdf_object_ref:e {#1/\int_eval:n{#2}}
+     } 
+   \cs_new:Npn \__kernel_pdf_object_id_indexed:nn #1 #2
+     {
+       \int_use:c
+         { c__pdf_object_ #1/\int_eval:n{#2} _int }       
+     }  
+ }
 
+%    \end{macrocode}
 % \subsection{a LastPage label}
 % See also issue \#2 in Accessible-xref
 % \begin{macro}{\@@_lastpagelabel:}