"[" Square bracket Curiosities in an array environment

Question

I am aware of the fact all of the following examples are in fact not the right way of doing whatever they do. But, I came across these examples by playing around with the code that WinEdt produces for inserting a cases environment.

Please consider the following MWE:

\documentclass{amsart} 
\begin{document}
\begin{eqnarray} 
\tau_G(g) = \left\{
            \begin{array}{ll}
            a & b \\
            [G & l
            \end{array}
            \right.
\end{eqnarray}
\end{document}

This one fails with the error, "Runaway Argument, missing { inserted".

However, the following does work:

\documentclass{amsart} 
\begin{document}
\begin{eqnarray} 
\tau_G(g) = \left\{
            \begin{array}{ll}
            [a & b \\
            %[G & l
            \end{array}
            \right.
\end{eqnarray}
\end{document}

I am unable to parse this error. When playing with some linear combination of these examples, I got the following error:

ERROR: File ended while scanning use of \@argarraycr.

Can someone shed some light on this, please?

Answer 1

\\ takes an optional argument, so the [ following \\ is taken to be the start of the optional argument. Just put the [ inside braces.

\documentclass{amsart} 
\begin{document}
\begin{eqnarray} 
\tau_G(g) = \left\{
            \begin{array}{ll}
            a & b \\
            {[}G & l
            \end{array}
            \right.
\end{eqnarray}
\end{document}