A modified section command

Question

I use the memoir \section command

\section[TOC title][Header title]{Main title}

together with a modified section command defined as

\newcommand{\optionalsection}[1]{%
\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
\section{#1}%
\renewcommand\thesection{\arabic{chapter}.\arabic{section}}%
}

that simply puts an asterisk in front of the section number as a mark that this is an optional section. The problem is that the command

\optionalsection[TOC title][Header title]{Main title}

does not work. Is is possible to fix this problem?

EDIT: The code of egreg works perfectly but the * symbol is displayed in the margin (see the attached figure)

Answer 1

You can use xparse to avoid clumsy redefinitions of the internals. There will be no problem with \textlatin and similar commands.

\documentclass{memoir}

\usepackage{xparse}
\usepackage{kantlipsum}

\NewDocumentCommand{\optionalsection}{O{#3}om}{%
  \renewcommand{\thesection}{\optasterisk\standardthesection}%
  \optzerotrue
  \IfNoValueTF{#2}{\section[#1]{#3}}{\section[#1][#2]{#3}}%
  \optzerofalse
  \renewcommand{\thesection}{\standardthesection}%
}
\AtBeginDocument{\let\standardthesection\thesection}

\NewDocumentCommand{\optasterisk}{}{%
  \ifoptzero\makebox[0pt][r]{*}\else*\fi
}
\newif\ifoptzero

\begin{document}

\frontmatter

\optzerotrue
\tableofcontents*
\optzerofalse

\mainmatter
\chapter{Test}

\section[TOC title][Header title]{Main title}

\optionalsection[TOC title opt][Header title opt]{Main title opt}\label{opt}

This is a reference to \ref{opt}.

\kant % to see the header title

\end{document}

This way the asterisk will have zero width in the toc and in the section titles, but will take its space in \ref and in headers.

Table of contents

Titles

Header

Answer 2

LAST EDIT RECOMMENDED AND WORKING SOLUTION:

It is based on my answer here: https://tex.stackexchange.com/a/380116/120578

\documentclass{memoir}
\usepackage[utf8]{inputenc}
\usepackage{xstring}
\usepackage{lipsum}
\usepackage[english,greek]{babel}

\makeatletter
\def\msection{%
\@ifstar{\@Starred}{\@nonStarred}%
}
\def\@Starred{%
\@ifnextchar[%
{\GenericWarning{}{Warning: A starred section can not have optional parameters. I am going to ignore them!}\@StarredWith}%
{\@StarredWithout}%
}      
\def\@StarredWith[#1]{%
\@ifnextchar[%
{\@StarredWithWith[#1]}
{\@StarredWithWithout[#1]}
}
\def\@StarredWithWithout[#1]#2{%
\ifx#1\empty\relax%
\else
\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
\fi
\section*[#1]{#2}%
}
\def\@StarredWithWith[#1][#2]#3{%
\ifx#1\empty\relax%
\else
\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
\fi
\section*[#1][#2]{#3}%
}
\def\@StarredWithout#1{%
\section*{#1}%
}
\def\@nonStarred{%
\@ifnextchar[%
{\@nonStarredWith}%
{\@nonStarredWithout}%
}
\def\@nonStarredWith[#1]{%
\@ifnextchar[%
{\@nonStarredWithWith[#1]}
{\@nonStarredWithWithout[#1]}%
}
\def\@nonStarredWithWith[#1][#2]#3{%
\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
\section[#1][#2]{#3}%
}
\def\@nonStarredWithWithout[#1]#2{%
\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
\section[#1]{#2}%
}
\def\@nonStarredWithout#1{%
\renewcommand\thesection{\arabic{chapter}.\arabic{section}}%
\section{#1}%
}
\makeatother 

\begin{document}
\tableofcontents

\chapter{\textlatin{Chapter}}
\msection{Εδώ είναι το \textlatin{Section} 1}
\lipsum[1-19]
\msection[ΠεριεχομεναΚαι\textlatin{HEADER}]{Δευτερη ενότητα Δηλ: \textlatin{No 2}}
\lipsum[1-8]
\msection[\textlatin{Test3HEADandTOC}]{Τριτη ενότητα}
\lipsum[1-8]
\msection[Περιεχομενα 4][\textlatin{TEST4HEAD}]{Τεστ \textlatin{Section} 4}
\lipsum[1-8]
\def\Sep{@}
\msection[Τεστ $\frac{5}{x}$,Περιεχομενα][\textlatin{TEST5HEAD}]{Τεστ \textlatin{Section} 5}
\lipsum[1-8]

\end{document}

(OLD) FIRST ANSWER (CAN NOT ACCEPT \textlatin inside the titles):

A solution with a change to the format of your command to: \msection[<TOC Title><Sep><HEADER Title>] where SEP is the separator that can be a comma (,) or whatever symbol you don't need in your titles.

\documentclass{memoir}
\usepackage{xstring}
\usepackage{lipsum}


\xdef\Sep{,}

\newcommand{\msection}[2][]{%
\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
\ifx#1\empty
\section{#2}
\else
\StrBefore{#1}{\Sep}[\First]
\StrBehind{#1}{\Sep}[\Second]
\ifx\First\empty
\ifx\Second\empty
\section{#2}
\else
\section[#2][\Second]{#2}
\fi
\else
\ifx\Second\empty
\section[\First]{#2}
\else
\section[\First][\Second]{#2}
\fi
\fi
\fi
}

\begin{document}
\tableofcontents

\chapter{Chapter}
\msection{Test Section 1}
\lipsum[1-19]
\msection[Test2TOCANDHEAD,]{Test Section 2}
\lipsum[1-8]
\msection[,Test3HEAD]{Test Section 3}
\lipsum[1-8]
\msection[Test4TOC,TEST4HEAD]{Test Section 4}
\lipsum[1-8]
\xdef\Sep{@}
\msection[TEST$\frac{5}{x}$,TOC@Test5,HEAD]{Test Section 5}
\lipsum[1-8]
\end{document}

Be careful to give every time the separator if you use an optional argument.

(OLD) SECOND ANSWER (Can Accept \textlatin inside the title but NOT inside the optional arguments/(the short titles) ):

New code:

\documentclass{memoir}
\usepackage{xstring}
\usepackage{lipsum}
\usepackage[english,greek]{babel}

\newsavebox{\myfbox}
\newsavebox{\mysbox}
\newsavebox{\mytbox}
\xdef\Sep{,}
\newcommand{\msection}[2][]{%
\savebox\mytbox{\hbox{#2}}
\ifx#1\empty
\section{\usebox{\mytbox}}
\else
\StrBefore{#1}{\Sep}[\First]
\savebox\myfbox{\hbox{\First}}
\StrBehind{#1}{\Sep}[\Second]
\savebox\mysbox{\hbox{\Second}}
\IfStrEq{\First}{\empty}
{  \IfStrEq{\Second}{\empty}
  {\section{\usebox{\mytbox}}
  }
  {\section[\usebox{\mytbox}][\usebox{\mysbox}]{\usebox{\mytbox}}
  }
}
{  \IfStrEq{\Second}{\empty}
  {\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
  \section[\usebox{\myfbox}]{\usebox{\mytbox}}
  }
  {
  \renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
  \section[\usebox{\myfbox}][\usebox{\mysbox}]{\usebox{\mytbox}}
  }
}
\fi
}

\begin{document}
\tableofcontents

\chapter{Chapter}
\msection{Test \textlatin{Section} 1}
\lipsum[1-19]
\msection[Test2TOCANDHEAD,]{Test Section 2}
\lipsum[1-8]
\msection[,Test3HEAD]{Test Section 3}
\lipsum[1-8]
\msection[Test4TOC,TEST4HEAD]{Test \textlatin{Section} 4}
\lipsum[1-8]
\xdef\Sep{@}
\msection[TEST$\frac{5}{x}$,TOC@%\foreignlanguage{english}{
TEST5HEAD%}
]{Test \textlatin{Section} 5}
\lipsum[1-8]
\end{document}

(OLD) THIRD ANSWER (A XeLaTeX working solution but the first in order (above) is better):

\documentclass{memoir}
\usepackage{xstring}
\usepackage{lipsum}
\usepackage{polyglossia}
\setmainlanguage{greek}
%\setotherlanguage{english}
%
\usepackage{xgreek}
\newfontfamily\greekfont{Linux Libertine O}


\xdef\Sep{,}
\newcommand{\msection}[2][]{%
\ifx#1\empty
\section{#2}
\else
\StrBefore{#1}{\Sep}[\First]
\StrBehind{#1}{\Sep}[\Second]
\IfStrEq{\First}{\empty}
{  \IfStrEq{\Second}{\empty}
  {\section{#2}
  }
  {\section[#2][\Second]{#2}
  }
}
{  \IfStrEq{\Second}{\empty}
  {\renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
  \section[\First]{#2}
  }
  {
  \renewcommand\thesection{*\arabic{chapter}.\arabic{section}}%
  \section[\First][\Second]{#2}
  }
}
\fi
}

\begin{document}
\tableofcontents

\chapter{Chapter}
\msection{Εδώ είναι το Section 1}
\lipsum[1-19]
\msection[ΠεριεχομεναΚαιHEADER,]{Δευτερη ενότητα Δηλ: Νο 2}
\lipsum[1-8]
\msection[,Test3HEAD]{Τριτη ενότητα}
\lipsum[1-8]
\msection[Περιεχομενα4,TEST4HEAD]{Τεστ Section 4}
\lipsum[1-8]
\def\Sep{@}
\msection[Τεστ $\frac{5}{x}$,Περιεχομενα@TEST5HEAD]{Τεστ Section 5}
\lipsum[1-8]
\end{document}