Case Change Equivs break in \text_titlecase_first:n

[moewew]

Via plk/biblatex#1246

A command declared with \DeclareCaseChangeEquivalent breaks in \text_titlecase_first:n (which has no LaTeX2e equivalent as far as I can see)

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}

\usepackage{etoolbox}

\newrobustcmd*{\foo}[1]{foo|#1|}
\newrobustcmd*{\fooA}[1]{A|#1|}
\newrobustcmd*{\fooB}[1]{B|#1|}
\newrobustcmd*{\fooC}[1]{C|#1|}
\newrobustcmd*{\fooD}[1]{D|#1|}

\DeclareCaseChangeEquivalent{\foo}{%
  \CaseSwitch
    {\fooA}
    {\fooB}
    {\fooC}
    {\fooD}}

\begin{document}
\MakeUppercase{\foo{bar} foo}

\MakeLowercase{\foo{bar} foo}

\MakeTitlecase{\foo{bar} foo}

\ExplSyntaxOn
\text_lowercase:n {\foo{bar} foo}\par

\text_uppercase:n {\foo{bar} foo}\par

\text_titlecase:n {\foo{bar} foo}\par

\text_titlecase_first:n {\foo{bar} foo}
\ExplSyntaxOff

\end{document}

gives

! Missing number, treated as zero.
<to be read again> 
                   \__text_change_case_switch_titleonly:nnNnnnn 
l.34 \text_titlecase_first:n {\foo{bar} foo}

tested with

LaTeX2e <2022-06-01> patch level 5
L3 programming layer <2022-08-23>

[PhelypeOleinik]

Providing the missing definition of \__text_change_case_switch_titleonly:nnNnnnn (seems like it should be equal to \__text_change_case_switch_title:nnNnnnn @josephwright?) should solve the problem:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{etoolbox}
\newrobustcmd*{\foo}[1]{foo|#1|}
\newrobustcmd*{\fooA}[1]{A|#1|}
\newrobustcmd*{\fooB}[1]{B|#1|}
\newrobustcmd*{\fooC}[1]{C|#1|}
\newrobustcmd*{\fooD}[1]{D|#1|}

\DeclareCaseChangeEquivalent{\foo}{%
  \CaseSwitch
    {\fooA}
    {\fooB}
    {\fooC}
    {\fooD}}

\begin{document}

\ExplSyntaxOn
\cs_new:Npn \__text_change_case_switch_titleonly:nnNnnnn #1#2#3#4#5#6#7
  {
    \__text_change_case_store:n {#7}
    \__text_change_case_loop:nnw {#1} {#2}
  }

\text_uppercase:n {\foo{bar} foo}\par

\text_lowercase:n {\foo{bar} foo}\par

\text_titlecase:n {\foo{bar} foo}\par

\text_titlecase_first:n {\foo{bar} foo}

\end{document}

[moewew]

That seems to work fine both in the example here as well as in the original biblatex MWE that sparked this report.