documentclass[tikz,border=2mm]{standalone}
usetikzlibrary{positioning, shapes.arrows, calc}

begin{document}

begin{tikzpicture}
node[draw] (a) {A};
node[draw, above right=2cm and 1cm of a] (b) {B};
path (a.north east) let p1=($(b.south west)-(a.north east)$) in
    node[single arrow, draw,
        minimum height={veclen(x1,y1)},
        shape border uses incircle,
        shape border rotate={atan2(y1,x1)},
        anchor=tail] at (a.north east) {};
end{tikzpicture}
end{document}

node[arrow between a and b] {};