Dear maintainer,

Your package fails to build from source against opencv 5.0.0+dfsg-1~exp1,
currently in experimental.

OpenCV 5.0 renames its entire install tree from opencv4 to opencv5 and ships
no compatibility shims:

  * pkg-config: only /usr/lib/<triplet>/pkgconfig/opencv5.pc.  There is no
                opencv4.pc and no opencv.pc.
  * headers:    /usr/include/opencv5/opencv2/..., no /usr/include/opencv4.
  * cmake:      /usr/lib/<triplet>/cmake/opencv5/OpenCVConfig.cmake.  A bare
                find_package(OpenCV) still resolves, but a pinned version
                such as find_package(OpenCV 4.7) no longer matches.
  * modules:    libopencv-calib3d-dev is split into libopencv-calib-dev,
                libopencv-geometry-dev, libopencv-stereo-dev and
                libopencv-ptcloud-dev; libopencv-features2d-dev becomes
                libopencv-features-dev.  The CMake component names change
                with them (features2d -> features).
  * geometry:   the new geometry module is not only a piece of calib3d.  It
                also absorbs shape and contour functions that used to live
                in imgproc -- contourArea, arcLength, approxPolyDP,
                boundingRect, minAreaRect, convexHull and
                getPerspectiveTransform among them -- and opencv2/opencv.hpp
                does not pull in opencv2/geometry.hpp, so code that reached
                those through the umbrella header now needs an explicit
                include of opencv2/geometry.hpp.

In uprightdiff's case the OpenCV 4 include directory is hardcoded in
debian/rules: override_dh_auto_configure runs

    dh_auto_configure -- CPPFLAGS="-I/usr/include/opencv4"

and that directory no longer exists, because OpenCV 5 installs its
headers under /usr/include/opencv5. The flag is still passed to every
g++ command, so the compiler never finds any OpenCV header and the first
translation unit stops at "fatal error: opencv2/core/core.hpp: No such
file or directory". The header itself is not the problem, the stale path
is. The cleanest fix is to drop that CPPFLAGS override and let the
OpenCV check in configure.ac query pkg-config (opencv5, optionally
falling back to opencv4) so the flags follow whichever version is
installed; modernising the aggregate opencv2/<module>/<module>.hpp
includes to opencv2/<module>.hpp is worth doing at the same time.

The same source builds fine in the same chroot against the OpenCV 4.10
packages currently in unstable, so this is specific to OpenCV 5.

The relevant part of the build log follows; the full log is attached.

     debian/rules override_dh_auto_configure
  make[1]: Entering directory '/build/reproducible-path/uprightdiff-1.4.0'
  dh_auto_configure -- CPPFLAGS="-I/usr/include/opencv4"
  ./configure --build=x86_64-linux-gnu --prefix=/usr
--includedir=\${prefix}/include --mandir=\${prefix}/share/man
--infodir=\${prefix}/share/info --sysconfdir=/etc --localstatedir=/var
--disable-option-checking --disable-silent-rules
--libdir=\${prefix}/lib/x86_64-linux-gnu --runstatedir=/run --disa
[...]
  checking for a BSD-compatible install... /usr/bin/install -c
  checking whether sleep supports fractional seconds... yes
  checking filesystem timestamp resolution... 0.01
  checking whether build environment is sane... yes
  checking for a race-free mkdir -p... /usr/bin/mkdir -p
  checking for gawk... no
  checking for mawk... mawk
  checking whether make sets $(MAKE)... yes
  checking whether make supports nested variables... yes
  checking xargs -n works... yes
  checking whether UID '999' is supported by ustar format... yes
  checking whether GID '999' is supported by ustar format... yes
  checking how to create a ustar tar archive... gnutar
  ./configure: line 3512: -std=c++11: command not found
  checking whether the C++ compiler works... yes
  checking for C++ compiler default output file name... a.out
  checking for suffix of executables...
  checking whether we are cross compiling... no
  checking for suffix of object files... o
  checking whether the compiler supports GNU C++... yes
  checking whether x86_64-linux-gnu-g++ accepts -g... yes
  checking whether make supports the include directive... yes (GNU style)
  checking dependency style of x86_64-linux-gnu-g++... none
  checking for gcc... x86_64-linux-gnu-gcc
  checking whether the compiler supports GNU C... yes
  checking whether x86_64-linux-gnu-gcc accepts -g... yes
  checking for x86_64-linux-gnu-gcc option to enable C23 features... none needed
  checking whether x86_64-linux-gnu-gcc understands -c and -o together... yes
  checking dependency style of x86_64-linux-gnu-gcc... none
  checking for main in -lboost_program_options... yes
  checking for main in -lopencv_core... yes
  checking for main in -lopencv_highgui... yes
  checking for main in -lopencv_imgproc... yes
  checking for main in -lopencv_imgcodecs... yes
  checking for stdio.h... yes
  checking for stdlib.h... yes
  checking for string.h... yes
  checking for inttypes.h... yes
  checking for stdint.h... yes
  checking for strings.h... yes
  checking for sys/stat.h... yes
  checking for sys/types.h... yes
  checking for unistd.h... yes
  checking for _Bool... no
  checking for stdbool.h that conforms to C99 or later... yes
  checking for inline... inline
  checking that generated files are newer than configure... done
  configure: creating ./config.status
  config.status: creating Makefile
  config.status: executing depfiles commands
  make[1]: Leaving directory '/build/reproducible-path/uprightdiff-1.4.0'
     dh_auto_build
  make -j40
  make[1]: Entering directory '/build/reproducible-path/uprightdiff-1.4.0'
  x86_64-linux-gnu-g++ -DPACKAGE_NAME=\"uprightdiff\"
-DPACKAGE_TARNAME=\"uprightdiff\" -DPACKAGE_VERSION=\"1.4.0\"
-DPACKAGE_STRING=\"uprightdiff\ 1.4.0\"
-DPACKAGE_BUGREPORT=\"https://phabricator.wikimedia.org/tag/uprightdiff/\"
-DPACKAGE_URL=\"\" -DPACKAGE=\"uprightdiff\" -DVERSION=\"1.4.0\"
-DHAVE [...]
  x86_64-linux-gnu-g++ -DPACKAGE_NAME=\"uprightdiff\"
-DPACKAGE_TARNAME=\"uprightdiff\" -DPACKAGE_VERSION=\"1.4.0\"
-DPACKAGE_STRING=\"uprightdiff\ 1.4.0\"
-DPACKAGE_BUGREPORT=\"https://phabricator.wikimedia.org/tag/uprightdiff/\"
-DPACKAGE_URL=\"\" -DPACKAGE=\"uprightdiff\" -DVERSION=\"1.4.0\"
-DHAVE [...]
  x86_64-linux-gnu-g++ -DPACKAGE_NAME=\"uprightdiff\"
-DPACKAGE_TARNAME=\"uprightdiff\" -DPACKAGE_VERSION=\"1.4.0\"
-DPACKAGE_STRING=\"uprightdiff\ 1.4.0\"
-DPACKAGE_BUGREPORT=\"https://phabricator.wikimedia.org/tag/uprightdiff/\"
-DPACKAGE_URL=\"\" -DPACKAGE=\"uprightdiff\" -DVERSION=\"1.4.0\"
-DHAVE [...]
  UprightDiff.cpp:1:10: fatal error: opencv2/core/core.hpp: No such
file or directory
      1 | #include <opencv2/core/core.hpp>
        |          ^~~~~~~~~~~~~~~~~~~~~~~
  compilation terminated.
  make[1]: *** [Makefile:390: UprightDiff.o] Error 1
  make[1]: *** Waiting for unfinished jobs....
  In file included from BlockMotionSearch.cpp:3:
  BlockMotionSearch.h:1:10: fatal error: opencv2/core/core.hpp: No
such file or directory
      1 | #include <opencv2/core/core.hpp>
        |          ^~~~~~~~~~~~~~~~~~~~~~~
  compilation terminated.
  make[1]: *** [Makefile:390: BlockMotionSearch.o] Error 1
  main.cpp:4:10: fatal error: opencv2/highgui/highgui.hpp: No such
file or directory
      4 | #include <opencv2/highgui/highgui.hpp>
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  compilation terminated.
  make[1]: *** [Makefile:390: main.o] Error 1
  make[1]: Leaving directory '/build/reproducible-path/uprightdiff-1.4.0'
  dh_auto_build: error: make -j40 returned exit code 2
  make: *** [debian/rules:5: binary] Error 25
  dpkg-buildpackage: error: debian/rules binary subprocess failed with
exit status 2

Please close this bug if it turns out to be a collision with another ongoing
transition rather than an OpenCV 5 problem.

Thanks,
Aron