documentclass{article}
makeatletter
defDrawPolygon(#1,#2){%
 begingroup
 draw(#1)
     foreach pt in {#2}{--(pt)}--cycle;%
 endgroup
}
makeatother

usepackage{tikz}


begin{document}
begin{tikzpicture}
    coordinate (A) at (4,0);
    coordinate (B) at (2,4);
    coordinate (C) at (0,0);
   % DrawPolygon(A,...,C) % error
    % now error undefinedifpgffor@alphabeticsequence else ifpgffor@assign@parse begingroup
    % ! File ended while scanning use of pgffor@@dotscharcheck.
    DrawPolygon(A,B,C)
end{tikzpicture}
end{document}