return of an access type from a function should be an implicit variable and not a constant type... here's why...

` $ vcom -2002 test1.vhd

** Error: test1.vhd(44): (vcom-1047) 
Actual (string literal) for class variable formal 
"path_in2" is not a variable.

** Error: test1.vhd(44): Function call returns 
type (error); expecting type std.TEXTIO.LINE.

** Note: test1.vhd(50): VHDL Compiler exiting
End time: 08:38:08 on Mar 23,2022, Elapsed time: 0:00:00
Errors: 2, Warnings: 0

use std.textio.all;

entity test1 is
end entity;

architecture beh of test1 is
    procedure join_path(
        variable path_in1 :in  line;
        variable path_in2 :in  line;
        variable path_out :out line         
    ) is
        variable mout :line;
        variable has_slash :boolean := false;
    begin
        path_out := null;
        
        if (path_in1 = null) then
            path_out := path_in2;
            return;
        end if;
        
        if (path_in1.all(path_in1'length-1) = '/' or
            path_in1.all(path_in1'length-1) = '\'
        ) then
            has_slash := true;
        end if;         
        write(mout, path_in1.all);
        if (not has_slash) then
            write(mout, '/');
        end if;
        write(mout, path_in2.all);
        path_out := mout;
        return;
    end procedure;

impure function to_line(constant s :string) 
return line is 
    variable m:line;
begin
    write(m, s);
    return m;
end function;

begin

process 
   variable projdir :line := null;
   variable mempath :line := null;
begin
   join_path(
       path_in1 => projdir,
       path_in2 => to_line("mem.txt"), --LINE 44
       path_out => mempath
   );

   report mempath.all;

   wait;
end process;

end architecture;

`