[Tex/LaTex] Changing the default series of the sans serif family only

Question

I'm using pdfLaTeX and a sans serif family that supports a light (l) face. I'd like to substitute the default medium (m) face by l only when \sffamily (or \textsf) is called.

In the next example, egreg's solution is only successfull in changing the \mddefault for the roman family, but not for sans serif.

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage{lmodern}

\makeatletter
\renewcommand{\mddefault}{\ifx\f@family\sfdefault sbc\else bx\fi}
\makeatother

\setlength\parskip{\baselineskip}\setlength\parindent{0pt}

\begin{document}

\textrm{The brown fox jumps over the lazy dog}                 \\
{\usefont{T1}{lmr}{m}{n}The brown fox jumps over the lazy dog} \\ % medium
{\usefont{T1}{lmr}{b}{n}The brown fox jumps over the lazy dog} \\ % bold
{\usefont{T1}{lmr}{bx}{n}The brown fox jumps over the lazy dog}   % bold expanded

\textsf{The brown fox jumps over the lazy dog}                   \\
{\usefont{T1}{lmss}{sbc}{n}The brown fox jumps over the lazy dog}\\ % semi bold condensed
{\usefont{T1}{lmss}{m}{n}The brown fox jumps over the lazy dog}  \\ % medium
{\usefont{T1}{lmss}{bx}{n}The brown fox jumps over the lazy dog}    % bold expanded

\end{document}

Then I tried to use \DeclareFontShape, but I always get the same type of error. For instance, using an example of pg 425 of The LaTeX Companion :

\documentclass{article}

\DeclareFontShape{0T1}{cmss}{m}{it}{ <-> sub * cmss/m/sl }{} 

\begin{document}
text
\end{document}

I get the error:

Answer 1

There are a number of issues to solve. The first is that redefining \mddefault in that way will not help, because \mddefault is not executed as part of \sffamily. The second is a traditional puzzle caused by some command being \long in a context and not \long in others.

A possible workaround is to redefine \curr@fontshape, that provides LaTeX with the current font attributes for later processing by NFSS.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{pdftexcmds}
\renewcommand{\sfdefault}{SourceSansPro-LF}

\makeatletter
\def\curr@fontshape{%
  \f@encoding/\f@family/%
  \ifnum\pdf@strcmp{\f@series}{\mddefault}=\z@
    \ifnum\pdf@strcmp{\f@family}{\sfdefault}=\z@
      l%
    \else
      \f@series
    \fi
  \else
    \f@series
  \fi/\f@shape}
\makeatother

\begin{document}
\sffamily abcdef

\rmfamily abcdef
\end{document}

I used Source Sans Pro because it has the "light" series. More tests are needed if other font attributes need to be changed depending on the current family.