biblatex-chicago (author-date) displays a redefined postnotedelim when only displaying the page number

Question

When using biblatex-chicago, if you change the \postnotedelim with a command like

\renewcommand*{\postnotedelim}{\addcolon\space}

and compile on Overleaf using TeX Live 2021, things don't turn out too well. Compiling the same code on TeX Live 2020 or previous works great.

Consider the following MWE:

\documentclass{article}

\usepackage[authordate]{biblatex-chicago}
\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@book{Jones2016,
    author = {Jones, John},
    title = {Title},
    date = {2016},
}

@book{Smith2018,
    author = {Smith, Sam},
    title = {Title},
    date = {2018},
}
\end{filecontents}

\addbibresource{\jobname.bib}

\renewcommand*{\postnotedelim}{\addcolon\space}

\begin{document}

\textcite[9]{Jones2016} discusses cool things. 
Some have commented that he's the best \autocite[40]{Smith2018}. 
But later in his work, Jones really goes off the rails \autocite[100]{Jones2016}. 
And by the end of the book, he's talking complete nonsense \autocite[500]{Jones2016}

\end{document}

In TeX Live 2020, everything works as expected:

However, this is what comes out when compiling with TeX Live 2021:

The standard first-mention \autocite command works fine with a postnote, but \textcite renders as Jones (2016: ), 9, and \autocite commands with a postnote that immediately follow a citation of the same work print the postnote delimiter before the page numbers in the parenthetical, instead of printing the page number in parentheses without the delimiter. Note that everything looks as expected with \renewcommand*{\postnotedelim}{\addcolon\addspace} commented out, regardless of which version of TeX Live I compile with:

I don't mind compiling with the 2020 edition of TeX Live, but is there a new "right way" of declaring a global \postnotedelim for biblatex-chicago on newer distributions of TeX Live and/or biblatex-chicago?

Answer 1

The actual definition of \postnotedelim for authordate in a current version of biblatex-chicago is a bit more complex than the simple \DeclareDelimFormat{postnotedelim}{\addcomma\space} in core biblatex. chicago-dates-common.cbx has

\renewcommand*{\postnotedelim}{% Cf. N&B style
  \ifboolexpr{%
    test {\ifciteibid}%
    and
    (
    test {\ifentrytype{jurisdiction}}%
    or
    test {\ifentrytype{legal}}%
    or
    test {\ifentrytype{legislation}}%
    )
  }%
  {\addspace}%
  {\iftoggle{cms@inlineibid}%
    {\togglefalse{cms@inlineibid}%
      \iffieldundef{prenote}% Bug fix
      {}%
      {\setunit{\cms@testspace}}}%
    {\iffieldequalstr{entrysubtype}{classical}% For Notes+Bib, too?
      {\DeclareNumChars*{abcdeABCDE:}%
        \iffieldpages{postnote}%
        {\setunit{\cms@testspace}}%
        {\newcunit}}%
      {\newcunit}\DeclareNumChars{.}}}}

where \newcunit is essentially \setunit{\addcomma\space}. So if we want colons, we need to replace the \newcunit by \setunit{\addcolon\space}.

\documentclass{article}

\usepackage[authordate]{biblatex-chicago}

\addbibresource{biblatex-examples.bib}

\makeatletter
\renewcommand*{\postnotedelim}{% Cf. N&B style
  \ifboolexpr{%
    test {\ifciteibid}%
    and
    (
    test {\ifentrytype{jurisdiction}}%
    or
    test {\ifentrytype{legal}}%
    or
    test {\ifentrytype{legislation}}%
    )
  }%
  {\addspace}%
  {\iftoggle{cms@inlineibid}%
    {\togglefalse{cms@inlineibid}%
      \iffieldundef{prenote}% Bug fix
      {}%
      {\setunit{\cms@testspace}}}%
    {\iffieldequalstr{entrysubtype}{classical}% For Notes+Bib, too?
      {\DeclareNumChars*{abcdeABCDE:}%
        \iffieldpages{postnote}%
        {\setunit{\cms@testspace}}%
        {\setunit{\addcolon\space}}}%
      {\setunit{\addcolon\space}}\DeclareNumChars{.}}}}
\makeatother

\begin{document}

\textcite[9]{sigfridsson} discusses cool things. 
Some have commented that he's the best \autocite[40]{worman}. 
But later in his work, Jones really goes off the rails \autocite[100]{sigfridsson}. 
And by the end of the book, he's talking complete nonsense \autocite[500]{sigfridsson}

\end{document}

Answer 2

Looking at the chicago-dates-common.cbx file that moewe's answer mentions and the \newcunit command in particular, it appears that the text component of \newcunit is defined by \newcunitpunct:

\protected\def\blx@newcunit{%
  \iftoggle{blx@keepunit}%
  {}%
  {\global\let\blx@unitpunct\newcunitpunct
    \global\toggletrue{blx@unit}}}%

\appto\blx@blxinit{%
  \let\newcunit\blx@newcunit}%

\newcommand*{\newcunitpunct}{\addcomma\space}

Adding \renewcommand*{\newcunitpunct}{\addcolon\space} to my preamble did exactly what I wanted.

Thanks again to moewe for pointing me to the appropriate file and the appropriate variable for me to get where I wanted to be.

Update: The above solution has unintended consequences (as @moewe correctly suggested it would), so my ultimate solution was basically to implement @moewe's answer with regexpatch just so it takes up fewer lines (though it probably introduces unnecessary complexity). I've put it here just for completeness's sake, but @moewe's answer seems to be the right way to do it.

\usepackage{regexpatch}

\makeatletter
\xpatchcmd*{\postnotedelim}{\newcunit }{\setunit{\addcolon \addspace} }{}{}
\makeatother

(Note that because there are two instances of \newcunit in the command, the starred version of \xpatchcmd replaces both occurrences of \newcunit.)

I'll mark @moewe's answer as correct, since it doesn't rely on any other packages, but just an explicit redefinition.