Problem with tcblisting: First listed LaTeX command is missing

Question

Following code shows the problem. I've declared a \newtcblisting with an optional argument.

\newtcblisting{codiltxb}[1][]{listing only, #1}

When the optional argument is present in further declarations, the listing is correct. (1st and 2nd boxes in figure)

\begin{codiltxb}[colback=green!20]
\Some code. Correct if the optional argument is present.
\end{codiltxb}

\begin{codiltxb}[]
\Some code. Correct if the optional argument, although empty, is present.
\end{codiltxb}

But if the optional argument is not present and the first word inside listing starts with \ (like \LaTeX commands), this first command disappears from resulting listing. (3rd box)

\begin{codiltxb}
\Some code. FAIL! No optional argument. First word (\LaTeX command) missing.
\end{codiltxb}

If the optional argument is not present and the first word doesn't start with \, the listing is correct again. (4th box)

\begin{codiltxb}
Some code. Correct! No optional argument. First word not \LaTeX command.
\end{codiltxb}

What's wrong with my code?

\documentclass{beamer}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[most]{tcolorbox}

\newtcblisting{codiltxb}[1][]{listing only, #1}

\begin{document}
\begin{frame}[fragile]
\frametitle{Listing}

\begin{codiltxb}[colback=green!20]
\Some code. Correct if the optional argument is present.
\end{codiltxb}

\begin{codiltxb}[]
\Some code. Correct if the optional argument, although empty, is present.
\end{codiltxb}

\begin{codiltxb}
\Some code. FAIL! No optional argument. First word (\LaTeX command) missing.
\end{codiltxb}

\begin{codiltxb}
Some code. Correct! No optional argument. First word not \LaTeX command.
\end{codiltxb}

\end{frame}

\end{document}

Note (2018-06-13): Although Thomas F. Sturm answer worked until now, some recent changes (2018-05-12) in xparse (see: xparse gobbles character(s) using an optional argument) make it fail again. A working solution while waiting for a better one is

\NewTCBListing{codiltxb}{ !O{} }{listing only, #1}

Answer 1

Update: !O{} instead of O{} inside \NewTCBListing{codiltxb}{ !O{} }{listing only, #1} to adapt to xparse changes with date of 2018-05-12.

Parsing problems with one optional argument only are known for \newtcblisting. The tcolorbox manual warns about it on page 245 (v2.60) and recommends to use \NewTCBListing instead.

The reason is the grabbing of the optional argument as egreg explained in his answer. It is done by the standard \@ifnextchar, if \newtcblisting is used. If \NewTCBListing is used, xparse code is used which is more suitable for the task.

So, replace

\newtcblisting{codiltxb}[1][]{listing only, #1}

by

\NewTCBListing{codiltxb}{ !O{} }{listing only, #1}

and you get the correct result.

\documentclass{beamer}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[most]{tcolorbox}

\NewTCBListing{codiltxb}{ !O{} }{listing only, #1}

\begin{document}
\begin{frame}[fragile]
\frametitle{Listing}

\begin{codiltxb}[colback=green!20]
\Some code. Correct if the optional argument is present.
\end{codiltxb}

\begin{codiltxb}[]
\Some code. Correct if the optional argument, although empty, is present.
\end{codiltxb}

\begin{codiltxb}
\Some code. FAIL! No optional argument. First word (\LaTeX command) missing.
\end{codiltxb}

\begin{codiltxb}
Some code. Correct! No optional argument. First word not \LaTeX command.
\end{codiltxb}

\end{frame}

\end{document}

Answer 2

This has to do with the grabbing of the optional argument; if I use \new@ifnextchar of amsmath and redefine \codiltxb the hard way, I get the expected result; this has the consequence that no space can intervene between the closing brace and the [, but should not be a big problem.

\documentclass{article}

\usepackage[T1]{fontenc} 
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[most]{tcolorbox}
\usepackage{amsmath}

\newtcblisting{codiltxb}[1][]{listing only, #1}

\makeatletter
\renewcommand{\codiltxb}{%
  \new@ifnextchar[\@codiltxb{\tcblisting {listing only,options@for=codiltxb}}%
}
\def\@codiltxb[#1]{\tcblisting {listing only,#1,options@for=codiltxb}}
\makeatother


\begin{document}

\begin{codiltxb}[colback=green!20]
\Some code. Correct if the optional argument is present.
\end{codiltxb}

\begin{codiltxb}[]
\Some code. Correct if the optional argument, although empty, is present.
\end{codiltxb}

\begin{codiltxb}
\Some code. FAIL! No optional argument. First word (\LaTeX command) missing.
\end{codiltxb}

\begin{codiltxb}
Some code. Correct! No optional argument. First word not \LaTeX command.
\end{codiltxb}

\end{document}