Modified Bourbaki problem

Question

The expex package has a nice macro \trailingcitation which typesets a citation at the right margin of the same line if there is space, or flushed right on the line if there is no space (first two examples below, using a simplified version of \trailingcitation). Unfortunately this macro cannot handle citations longer than a single line (third example).

I tried to use Peter Wilson's \atright, but it puts as little text of the citation as possible on the first line (fourth example).

What I want instead is that if the citation is longer than a single line, it starts on a new line, and is flushed right (or, ideally, justified but with the last line aligned with the right rather than the left margin) (fifth example). How do I accomplish this?

---

\documentclass[draft]{article}
\usepackage[a4paper,margin=5cm]{geometry}
\usepackage{parskip}
\def\trailingcitation#1{% adapted from expex
   \hskip 1.5em plus 1fill
   \penalty100\null\nobreak \hskip 0pt plus 1fill
   \hbox{#1}}
\newcommand*{\atright}[1]{{%
  \unskip\nobreak\hfil\penalty50
  \hskip2em\hbox{}\nobreak\hfil#1
  \parfillskip=0pt\finalhyphendemerits=0\par}}
\begin{document}
A short quote gets a citation on the same line.
\trailingcitation{(Smith 2015)}
When the citation does not fit on the same line, a new line is used.
\trailingcitation{(Smith 2015)}
Due to \verb$\hbox$, long citations cause an overfull hbox.
\trailingcitation{(Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)}
\verb$\atright$ will make the first line as short as possible:
\atright{(Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)}
Whereas I want something like this (but only if it does not fit on the same line):\par\vspace{-1ex}
{\raggedleft (Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)\par}
\end{document}

Answer 1

\documentclass[draft]{article}
\usepackage[a4paper,margin=5cm]{geometry}
\usepackage{parskip}
\def\trailingcitation#1{% adapted from expex
   \unskip
   \hskip 1.5em plus 1fill
   \penalty100\null\nobreak \hskip 0pt plus 1fill
   \discretionary
   {}%
   {\vtop{%
  % shorter line (if you like)
  \leftskip 20pt
  #1\par
  % reset last line flush right
  \setbox0\lastbox
  \nointerlineskip
  \hbox to \hsize{\hfill\unhbox0\unskip\unskip}}%
    }%
   {\hbox{#1}}%
}
\begin{document}
A short quote gets a citation on the same line.
\trailingcitation{(Smith 2015)}
When the citation does not fit on the same line, a new line is used.
\trailingcitation{(Smith 2015)}
Due to \verb$\hbox$, long citations cause an overfull hbox.
\trailingcitation{(Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)}
\end{document}

In lists and ExPex examples, a limited amount of horizontal space is available. The \vtop needs to be set to \hsize=\linewidth for lists, and for ExPex examples the remaining width can be computed. The following works in both ExPex examples and lists. It also uses ExPex's mincitesep and everytrailingcitation, which were removed from the MWE in the question for simplicity:

\documentclass[draft]{article}
\usepackage[a4paper,margin=5cm]{geometry}
\usepackage{expex}
% Alan Munn's method to get remaining space in expex examples, https://tex.stackexchange.com/a/376534:
\usepackage{calc}
\usepackage{etoolbox}
\newlength{\remaining}
\newcommand{\remainpex}{%
   \setlength{\remaining}{\linewidth-\lingtextoffset-\linglabelwidth-
      \lingnumoffset-\linglabeloffset-\widthof{\exnoprint}}%
}
\newcommand{\remainex}{%
   \setlength{\remaining}{\linewidth-\lingnumoffset-
      \lingtextoffset-\widthof{\exnoprint}}%
}
% We also need to know whether we are in an example or not:
\newif\ifinex \inexfalse
\pretocmd{\pex}{\remainpex\inextrue}{}{}
\pretocmd{\ex}{\remainex\inextrue}{}{}
\apptocmd{\xe}{\inexfalse}{}{}
\makeatletter
\def\trailingcitation#1{% adapted from expex
   \unskip
   \hskip\ling@mincitesep plus 1fill
   \penalty100\null\nobreak \hskip 0pt plus 1fill
   \discretionary
   {}%
   {\vtop{%
  \hsize=\ifinex\remaining\else\linewidth\fi
  % shorter line (if you like)
  \leftskip 20pt
  \ling@everytrailingcitation #1\par
  % reset last line flush right
  \setbox0\lastbox
  \nointerlineskip
  \hbox to \hsize{\hfill\unhbox0\unskip\unskip}}%
    }%
   {\hbox{#1}}%
}
\makeatother
\begin{document}
\ex
  Due to \verb$\hbox$, long citations cause an overfull hbox.
  \trailingcitation{(Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)}
\xe
\pex
\a
  Due to \verb$\hbox$, long citations cause an overfull hbox.
  \trailingcitation{(Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)}
\xe
\begin{itemize}
  \item
    Due to \verb$\hbox$, long citations cause an overfull hbox.
    \trailingcitation{(Author, \textit{Some very long title of a secondary source}, in citation by Smith 2015)}
\end{itemize}
\end{document}