Fix for https://tex.stackexchange.com/a/89840/250119

a.tex

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{array}
\usepackage{makecell}
\newcolumntype{x}[1]{>{\centering\arraybackslash}p{#1}}
\usepackage{tikz}

\begin{document}
\newcommand\diag[4]{%
  \multicolumn{1}{@{}p{#2}@{}|}{%
  \hsize=\dimexpr#2+2\tabcolsep\relax
  $\vcenter{\begin{tikzpicture}[baseline=0,anchor=south west,inner sep=#1]
  \path[use as bounding box] (0,0) rectangle (#2+2\tabcolsep,\baselineskip);
  \node[minimum width={#2+2\tabcolsep-\pgflinewidth},
        minimum height=\baselineskip+\extrarowheight-\pgflinewidth] (box) {};
  \draw[line cap=round] (box.north west) -- (box.south east);
  \node[anchor=south west] at (box.south west) {#3};
  \node[anchor=north east] at (box.north east) {#4};
\end{tikzpicture}}$}}
\setlength{\extrarowheight}{0.1cm}
\begin{tabular}{|x{0.5cm}|x{0.5cm}|x{0.5cm}|x{0.5cm}|x{0.5cm}|}\hline
&&&&20\\ \hline
&&&&30\\ \hline
&&&&45\\ \hline
?5&12&18&50&\diag{.1em}{0.5cm}{$a_i$}{$b_j$}\\ \hline
\end{tabular}

\end{document}