libxapian-dev needs to be rebuilt with GCC 11 in impish, otherwise it emits warnings when you try to compile other things with the now-default GCC 11 with it:
...
[9/61] c++ -Itest/library.p -I/usr/include/x86_64-linux-gnu -I/usr/include/p11-kit-1 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Werror -std=c++11 -g -O2 '-ffile-prefix-map=/<<BUILDDIR>>/libkiwix-10.0.0+git202108071333.83e757a~impish=.' -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DGTEST_HAS_PTHREAD=1 -pthread -MD -MQ test/library.p/library.cpp.o -MF test/library.p/library.cpp.o.d -o test/library.p/library.cpp.o -c ../test/library.cpp
[10/61] c++ -Isrc/libkiwix.so.10.0.0.p -Isrc -I../src -Iinclude -I../include -I/usr/include/kainjow -Istatic -I/usr/include/x86_64-linux-gnu -I/usr/include/p11-kit-1 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Werror -std=c++11 -g -O2 '-ffile-prefix-map=/<<BUILDDIR>>/libkiwix-10.0.0+git202108071333.83e757a~impish=.' -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -pthread -MD -MQ src/libkiwix.so.10.0.0.p/library.cpp.o -MF src/libkiwix.so.10.0.0.p/library.cpp.o.d -o src/libkiwix.so.10.0.0.p/library.cpp.o -c ../src/library.cpp
FAILED: src/libkiwix.so.10.0.0.p/library.cpp.o
c++ -Isrc/libkiwix.so.10.0.0.p -Isrc -I../src -Iinclude -I../include -I/usr/include/kainjow -Istatic -I/usr/include/x86_64-linux-gnu -I/usr/include/p11-kit-1 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Werror -std=c++11 -g -O2 '-ffile-prefix-map=/<<BUILDDIR>>/libkiwix-10.0.0+git202108071333.83e757a~impish=.' -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -pthread -MD -MQ src/libkiwix.so.10.0.0.p/library.cpp.o -MF src/libkiwix.so.10.0.0.p/library.cpp.o.d -o src/libkiwix.so.10.0.0.p/library.cpp.o -c ../src/library.cpp
In file included from /usr/include/xapian.h:44,
from ../src/library.cpp:35:
/usr/include/xapian/version.h:29:2: error: #warning The C++ ABI version of compiler you are using does not exactly match [-Werror=cpp]
29 | #warning The C++ ABI version of compiler you are using does not exactly match
| ^~~~~~~
/usr/include/xapian/version.h:30:2: error: #warning that of the compiler used to build the library. If linking fails [-Werror=cpp]
30 | #warning that of the compiler used to build the library. If linking fails
| ^~~~~~~
/usr/include/xapian/version.h:31:2: error: #warning due to missing symbols, this is probably the reason why. [-Werror=cpp]
31 | #warning due to missing symbols, this is probably the reason why.
| ^~~~~~~
/usr/include/xapian/version.h:32:2: error: #warning The Xapian library was built with g++ 10.2.1 [-Werror=cpp]
32 | #warning The Xapian library was built with g++ 10.2.1
| ^~~~~~~
cc1plus: all warnings being treated as errors
[11/61] c++ -Isrc/libkiwix.so.10.0.0.p -Isrc -I../src -Iinclude -I../include -I/usr/include/kainjow -Istatic -I/usr/include/x86_64-linux-gnu -I/usr/include/p11-kit-1 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Werror -std=c++11 -g -O2 '-ffile-prefix-map=/<<BUILDDIR>>/libkiwix-10.0.0+git202108071333.83e757a~impish=.' -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -pthread -MD -MQ src/libkiwix.so.10.0.0.p/downloader.cpp.o -MF src/libkiwix.so.10.0.0.p/downloader.cpp.o.d -o src/libkiwix.so.10.0.0.p/downloader.cpp.o -c ../src/downloader.cpp
[12/61] c++ -Isrc/libkiwix.so.10.0.0.p -Isrc -I../src -Iinclude -I../include -I/usr/include/kainjow -Istatic -I/usr/include/x86_64-linux-gnu -I/usr/include/p11-kit-1 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Werror -std=c++11 -g -O2 '-ffile-prefix-map=/<<BUILDDIR>>/libkiwix-10.0.0+git202108071333.83e757a~impish=.' -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -pthread -MD -MQ src/libkiwix.so.10.0.0.p/reader.cpp.o -MF src/libkiwix.so.10.0.0.p/reader.cpp.o.d -o src/libkiwix.so.10.0.0.p/reader.cpp.o -c ../src/reader.cpp
[13/61] c++ -Isrc/libkiwix.so.10.0.0.p -Isrc -I../src -Iinclude -I../include -I/usr/include/kainjow -Istatic -I/usr/include/x86_64-linux-gnu -I/usr/include/p11-kit-1 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Werror -std=c++11 -g -O2 '-ffile-prefix-map=/<<BUILDDIR>>/libkiwix-10.0.0+git202108071333.83e757a~impish=.' -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -pthread -MD -MQ src/libkiwix.so.10.0.0.p/opds_dumper.cpp.o -MF src/libkiwix.so.10.0.0.p/opds_dumper.cpp.o.d -o src/libkiwix.so.10.0.0.p/opds_dumper.cpp.o -c ../src/opds_dumper.cpp
ninja: build stopped: subcommand failed.
dh_auto_build: error: cd obj-x86_64-linux-gnu && LC_ALL=C.UTF-8 ninja -j4 -v returned exit code 1
make: *** [debian/rules:5: binary] Error 25
dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
I verified that simply rebuilding xapian-core with GCC 11 makes this problem go away.
Cause problems in Kiwix/openZIM PPA PPA CD, see https:/