With xelatex + color package, \color_ensure_current: leaves 1 in input stream

[muzimuzhi]

Example:

% !TeX TS-program = xelatex
\documentclass{article}
\usepackage{color}

\begin{document}
\ExplSyntaxOn

\hbox_set:Nn \l_tmpa_box
  { \color_ensure_current: }
\box_use:N \l_tmpa_box

\ExplSyntaxOff
\end{document}

• Expected: Box \l_tmpa_box is empty
• Actual: it contains a character 1.

Analysis:

With xelatex + color package, initially \current@color holds ␣Black (a normal space followed by word Black). \color_ensure_current: is defined as

\cs_new_protected:Npn \color_ensure_current:
  {
    \__color_backend_pickup:N \l__color_current_tl
    \__color_select:N \l__color_current_tl
  }

• Before 94aa846 in Enable creation of new color models #757, \__color_backend_pickup:N \l__color_current_tl will set \l__color_current_tl to be {spot}{Black 1}.
• But now (since l3backend 2020-08-07 I think), \__color_current_tl is set to be {Black}{1}. Since Black is an unknown color model, \__color_select:nn {Black}{1}, the intermediate expansion result of \__color_select:N \l__color_current_tl, leaves 1 in input stream.