私の使用している環境を示し損ねていてすみません.
Cygwin で最新版にアップデートしてから実行してみましたが Segmentation fault でした.
$ uname -svrmopi; bash --version | head -1; printf '%s\n' '\indexentry{entryA}{0}' '\indexentry{entryA}{-1.1.1}' | mendex -s <( printf '%s\n' 'page_compositor "."' ); printf '%s\n' "${PIPESTATUS[@]}"
CYGWIN_NT-10.0-26100 3.6.4-1.x86_64 2025-07-15 07:55 UTC x86_64 unknown unknown Cygwin
GNU bash, version 5.2.21(1)-release (x86_64-pc-cygwin)
This is mendex version 3.8 [09-Feb-2025] (utf8.uptex) (TeX Live 2025).
Scanning style file /dev/fd/63....done.
Scanning input file stdin....done (2 entries accepted, 0 rejected).
2 entries accepted, 0 rejected.
Sorting index....done(0 comparisons).
Sorting pages....done(2 comparisons).
Making index file.\begin{theindex}
Segmentation fault
0
139
$ uname -svrmopi; bash --version | head -1; printf '%s\n' '\indexentry{entryA}{0}' '\indexentry{entryA}{-1.1.1}' | upmendex -s <( printf '%s\n' 'page_compositor "."' ); printf '%s\n' "${PIPESTATUS[@]}"
CYGWIN_NT-10.0-26100 3.6.4-1.x86_64 2025-07-15 07:55 UTC x86_64 unknown unknown Cygwin
GNU bash, version 5.2.21(1)-release (x86_64-pc-cygwin)
This is upmendex version 1.11 [ICU 76.1] (TeX Live 2025).
Scanning style file /dev/fd/63....done.
Scanning input file stdin....done (2 entries accepted, 0 rejected).
2 entries accepted, 0 rejected.
Sorting index....done(0 comparisons).
Sorting pages....done(2 comparisons).
Making index file.\begin{theindex}
Segmentation fault
0
139
$
そのほか,makeindex の挙動も不思議でした.通常はありえない入力に対してですが……
やはり Segmentation fault や,ページ番号のソート順がおかしかったりします.
$ uname -svrmopi; bash --version | head -1; printf '%s\n' '\indexentry{entryA}{0}' '\indexentry{entryA}{1234567890123456789}' '\indexentry{entryA}{12345678901234567890}' | makeindex
CYGWIN_NT-10.0-26100 3.6.4-1.x86_64 2025-07-15 07:55 UTC x86_64 unknown unknown Cygwin
GNU bash, version 5.2.21(1)-release (x86_64-pc-cygwin)
This is makeindex, version 2.17 [TeX Live 2025] (kpathsea + Thai support).
Scanning input file stdin....done (3 entries accepted, 0 rejected).
Sorting entries....Segmentation fault
$ uname -svrmopi; bash --version | head -1; printf '%s\n' '\indexentry{entryA}{0}' '\indexentry{entryA}{12345678901234567890}' | makeindex
CYGWIN_NT-10.0-26100 3.6.4-1.x86_64 2025-07-15 07:55 UTC x86_64 unknown unknown Cygwin
GNU bash, version 5.2.21(1)-release (x86_64-pc-cygwin)
This is makeindex, version 2.17 [TeX Live 2025] (kpathsea + Thai support).
Scanning input file stdin....done (2 entries accepted, 0 rejected).
Sorting entries....done (2 comparisons).
Generating output file stdout...\begin{theindex}
.
\item entryA, 12345678901234567890, 0
\end{theindex}
done (5 lines written, 0 warnings).
Output written in stdout.
Transcript written in stderr.
$