Overriding `\bfseries` and `\itshape` to do special things for certain families

Question

My problem arises from trying to work around the issue described in question 26276: I want to use SizeFeatures to explicitly select another family for small sizes, but this causes bold / italic / etc. to stop working in small sizes. My solution was to redefine \bfseries and \itshape to actually use a new font face for each variant, but this workaround is applied no matter what family is actually in use ... which causes problems if I want to use more than one (real) font family in my document.

Question. How do I reliably test what the current font family is, so that my code can decide whether to apply the workaround or not?

Here some sample code:

\documentclass[12pt,a4paper]{article}
\usepackage{fontspec}
\usepackage{etoolbox}
\setmainfont[%
SizeFeatures={%
  {Size=-10, Font={Georgia}}, %
  {Size=10-}%
}]{Times}

\newfontface\boldface[%
SizeFeatures={%
  {Size=-10, Font={Georgia Bold}}, %
  {Size=10-}%
}]{Times Bold}

\newfontface\italicface[%
SizeFeatures={%
  {Size=-10, Font={Georgia Italic}}, %
  {Size=10-}%
}]{Times Italic}

\newfontface\bolditalicface[%
SizeFeatures={%
  {Size=-10, Font={Georgia Bold Italic}}, %
  {Size=10-}%
}]{Times Bold Italic}

\setsansfont{Helvetica}

\DeclareTextFontCommand{\textbfit}{\bfseries\itshape}

\ExplSyntaxOn
\makeatletter
\let\int@bfseries\bfseries
\let\int@itshape\itshape

\renewcommand{\bfseries}{\str_if_eq_x:nnTF \f@shape \itdefault
  {\bolditalicface \int@bfseries\int@itshape}
  {\boldface \int@bfseries}
}
\renewcommand{\itshape}{\str_if_eq_x:nnTF \f@series \bfdefault
  {\bolditalicface \int@bfseries\int@itshape}
  {\italicface \int@itshape}
}
\makeatother
\ExplSyntaxOff

\begin{document}
\noindent
{\normalfont Roman: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}} \newline
{\scriptsize Roman: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}} \newline
{\sffamily Sans: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}} \newline
{\sffamily\scriptsize Sans: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}} \newline
\end{document}

Answer 1

I just found out I can use xstring to test for string prefixes. This version of my workaround seems more robust:

\documentclass[12pt,a4paper]{article}
\usepackage{fontspec}
\usepackage{xstring}
\setmainfont[%
SizeFeatures={%
  {Size=-10, Font={Georgia}}, %
  {Size=10-}%
}]{Times}

\newfontfamily\boldface[%
BoldFont={Times Bold}, %
SizeFeatures={%
  {Size=-10, Font={Georgia Bold}}, %
  {Size=10-}%
}]{Times Bold}

\newfontfamily\italicface[%
ItalicFont={Times Italic}, %
SizeFeatures={%
  {Size=-10, Font={Georgia Italic}}, %
  {Size=10-}%
}]{Times Italic}

\newfontfamily\bolditalicface[%
BoldItalicFont={Times Bold Italic}, %
SizeFeatures={%
  {Size=-10, Font={Georgia Bold Italic}}, %
  {Size=10-}%
}]{Times Bold Italic}

\setsansfont{Helvetica}

\DeclareTextFontCommand{\textbfit}{\bfseries\itshape}

\ExplSyntaxOn
\makeatletter
\let\int@bfseries\bfseries
\let\int@itshape\itshape
\let\int@upshape\upshape

\renewcommand{\bfseries}{\IfBeginWith{\f@family}{Times}
  {\str_if_eq_x:nnTF \f@shape \itdefault
    {\bolditalicface \int@bfseries\int@itshape}
    {\boldface \int@bfseries}
  }{\int@bfseries}
}
\renewcommand{\itshape}{\IfBeginWith{\f@family}{Times}
  {\str_if_eq_x:nnTF \f@series \bfdefault
    {\bolditalicface \int@bfseries\int@itshape}
    {\italicface \int@itshape}
  }{\int@itshape}
}
\renewcommand{\upshape}{\IfBeginWith{\f@family}{Times}
  {\str_if_eq_x:nnTF \f@series \bfdefault
    {\boldface \int@bfseries\int@upshape}
    {\normalfont \int@upshape}
  }{\int@itshape}
}

\let\emshape\itshape
\let\eminnershape\upshape
\makeatother
\ExplSyntaxOff

\begin{document}
\emph{Emphasis with \emph{inner emphasis and \emph{inner-inner emphasis}}, plus {\upshape upright}.}

{\scriptsize \emph{Emphasis with \emph{inner emphasis and \emph{inner-inner emphasis}}, plus {\upshape upright}.}}

\bigskip\noindent
{\normalfont Roman: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}}

\noindent
{\scriptsize Roman: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}}

\noindent
{\sffamily Sans: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}}

\noindent
{\sffamily\scriptsize Sans: Normal \textit{Italic} \textbf{Bold} \textbfit{Bold Italic} \textit{\bfseries Italic Bold}}
\end{document}