Command \nobreakspace unavailable when switching to T1 encoding under XeLaTeX

Question

I'd like to use the luximono face for monospaced typesetting under XeLaTeX. Since it is already installed in the MikTeX distribution under T1 encoding, I switch to the latter when I need monospaced type. In order to do this, I have defined corresponding commands, as shown in the MWE below. Now, if I do not redefine the control sequence \nobreakspace in a form such as, e.g., I have done for my MWE, I get the error message Command \nobreakspace unavailable in encoding T1. My redefiniton is a workaround to this problem, and it has to it more of a patch than of a satisfactory solution. Could anyone possibly explain what is going on and how to solve it in a more appropriate way?

\documentclass[11pt]{memoir}

\usepackage[T1]{fontenc}
\usepackage{mathpazo}
\usepackage{fontspec}
\usepackage{xunicode}

\defaultfontfeatures{Numbers=OldStyle,Ligatures=TeX}
\setmainfont[BoldFont=texgyrepagella-bold.otf,
             ItalicFont=texgyrepagella-italic.otf,
             BoldItalicFont=texgyrepagella-bolditalic.otf]{texgyrepagella-regular.otf}

\usepackage{microtype}

\newcommand{\xlxmono}{
            \def\nobreakspace{\nobreak\space\nobreak}
            \fontfamily{ul9}\fontencoding{T1}
            \selectfont
           }
\renewcommand*{\ttfamily}{\xlxmono}
\renewcommand*{\texttt}[1]{
            {\xlxmono #1}
           }

\begin{document}

    {\ttfamily This is one line set in a monotype face with a nobreakspace command~here.}

    \texttt{This makes for yet another line with~it.}

\end{document}

Answer 1

If you modify your example to

\show\nobreakspace
\usepackage{fontspec}
\show\nobreakspace

You will see

> \nobreakspace=macro:
->\protect \nobreakspace  .
l.9 \show\nobreakspace

...

> \nobreakspace=macro:
->\EU1-cmd \nobreakspace \EU1\nobreakspace .
l.11 \show\nobreakspace

which means that the EU1 encoding setup has changed \nobreakspace from being an encoding-independent command to an encoding-dependent command, but without setting up a default definition so it works in all encodings. Re-instating the original definition as the default is probably the simplest thing, however note you should not be mixing fontenc and fontspec encoding definitions in a single document.

The original definition is

\DeclareRobustCommand{\nobreakspace}{%
   \leavevmode\nobreak\ }

So you could declare a default via

 \DeclareTextCommandDefault{\nobreakspace}{\leavevmode\nobreak\ } 

as noted in the comments.

Note that your definition would not work if it was used in vertical mode, eg the start of a paragraph as in that position the \nobreak would prevent a page break not a line break.

Unrelated to the question you have lots of white space tokens in your definition which will produce white space in the output: put a % at the end of every line within the definitiosn that ends with } or {.

Note also you need to use xelatex to use fontspec but you also load microtype which generates a warning that it does not work with xelatex:

Package microtype Warning: You don't seem to be using pdftex.
(microtype)                `microtype' only works with pdftex.
(microtype)                Try running `pdflatex' instead of `xelatex'.

Unless you have the beta version as pointed out by @UlrikeFischer. See. http://tlcontrib.metatex.org/cgi-bin/package.cgi/action=view/id=423

Answer 2

I'd simply provide a suitable definition for \nobreakspace in the T1 encoding and define \ttfamily so that it chooses both LuxiMono and the T1 encoding:

\usepackage{xpatch}
\renewcommand{\ttdefault}{ul9}
\xpatchcmd{\ttfamily}{\selectfont}{\fontencoding{T1}\selectfont}{}{}
\DeclareTextCommand{\nobreakspace}{T1}{\leavevmode\nobreak\ }

Since \texttt relies on \ttfamily, it's not necessary to act on it.

Answer 3

use this preamble:

\documentclass[11pt]{memoir}

\usepackage{fontspec}
\usepackage{unicode-math}
\usepackage{microtype}

\defaultfontfeatures{Numbers=OldStyle,Ligatures=TeX}
\setmainfont[BoldFont=texgyrepagella-bold.otf,
             ItalicFont=texgyrepagella-italic.otf,
             BoldItalicFont=texgyrepagella-bolditalic.otf]{texgyrepagella-regular.otf}

\newcommand\xlxmono{%
            \def\nobreakspace{\nobreak\space\nobreak}
            \fontfamily{ul9}\fontencoding{T1}\selectfont}
\renewcommand*\ttfamily{\xlxmono}
\renewcommand*\texttt[1]{{\xlxmono #1}}
\begin{document}