Hi,

taskflow fails to build in a test rebuild on at least amd64 and arm64 with
gcc-16/g++-16, but builds properly with gcc-15/g++-15. The severity of this
report will be raised before the forky release.

The full build log can be found at:
https://people.debian.org/~ema/gcc-16-rebuilds/output-1/taskflow_arm64.build.xz

The last lines of the build log are at the end of this report.

To build with GCC 16, either set CC=gcc-16 CXX=g++-16 explicitly,
or install the gcc, g++, gfortran, ... packages from experimental.

  apt-get -t=experimental install g++

Common build failures include unused (but set) variables, array subscripts
partly outside array bounds, and new/dropped symbols in Debian symbols files.
For other C/C++ related build failures see the porting guide at
http://gcc.gnu.org/gcc-16/porting_to.html

Please only close this issue after double-checking that the package can be
built correctly with GCC 16.

Please do not reassign this bug to another package. If a fix in another package
is required, then file a bug for the other package (or clone), and mark this
bug as blocked by the bug in the other package.

[...]


[doctest] test cases: 1 | 0 passed | 1 failed | 83 skipped
[doctest] assertions: 3 | 2 passed | 1 failed |
[doctest] Status: FAILURE!

    Start 954: InclusiveScan.string.4threads
4/6 Test #954: InclusiveScan.string.4threads ....***Failed    0.00 sec
[doctest] doctest version is "2.4.12"
[doctest] run with "--help" for options
===============================================================================
./unittests/test_scan.cpp:129:
TEST CASE:  InclusiveScan.string.4threads

./unittests/test_scan.cpp:61: FATAL ERROR: REQUIRE( input == golden ) is NOT correct!
  values: REQUIRE( {?} == {?} )

===============================================================================
[doctest] test cases: 1 | 0 passed | 1 failed | 83 skipped
[doctest] assertions: 3 | 2 passed | 1 failed |
[doctest] Status: FAILURE!

    Start 955: InclusiveScan.string.8threads
5/6 Test #955: InclusiveScan.string.8threads ....***Failed    0.01 sec
[doctest] doctest version is "2.4.12"
[doctest] run with "--help" for options
===============================================================================
./unittests/test_scan.cpp:133:
TEST CASE:  InclusiveScan.string.8threads

./unittests/test_scan.cpp:61: FATAL ERROR: REQUIRE( input == golden ) is NOT correct!
  values: REQUIRE( {?} == {?} )

===============================================================================
[doctest] test cases: 1 | 0 passed | 1 failed | 83 skipped
[doctest] assertions: 3 | 2 passed | 1 failed |
[doctest] Status: FAILURE!

    Start 956: InclusiveScan.string.12threads
6/6 Test #956: InclusiveScan.string.12threads ...***Failed    0.01 sec
[doctest] doctest version is "2.4.12"
[doctest] run with "--help" for options
===============================================================================
./unittests/test_scan.cpp:137:
TEST CASE:  InclusiveScan.string.12threads

./unittests/test_scan.cpp:61: FATAL ERROR: REQUIRE( input == golden ) is NOT correct!
  values: REQUIRE( {?} == {?} )

===============================================================================
[doctest] test cases: 1 | 0 passed | 1 failed | 83 skipped
[doctest] assertions: 3 | 2 passed | 1 failed |
[doctest] Status: FAILURE!


0% tests passed, 6 tests failed out of 6

Total Test time (real) =   0.07 sec

The following tests FAILED:
	951 - InclusiveScan.string.1thread (Failed)
	952 - InclusiveScan.string.2threads (Failed)
	953 - InclusiveScan.string.3threads (Failed)
	954 - InclusiveScan.string.4threads (Failed)
	955 - InclusiveScan.string.8threads (Failed)
	956 - InclusiveScan.string.12threads (Failed)
Errors while running CTest
make[2]: *** [Makefile:74: test] Error 8
make[2]: Leaving directory '/build/reproducible-path/taskflow-3.9.0+ds/buildtest'
make[1]: *** [debian/rules:67: override_dh_auto_test-arch] Error 2
make[1]: Leaving directory '/build/reproducible-path/taskflow-3.9.0+ds'
make: *** [debian/rules:15: binary] Error 2
dpkg-buildpackage: error: debian/rules binary subprocess failed with exit status 2

Hi Emanuele,
packages and I saw no one has replied so thought I'd take a look.

I've tracked this down and it isn't a bug in taskflow; it appears to be a
regression in libstdc++ 16.

Now I am no compiler expert so please take what I say with a pinch of salt.

The no-init overload std::inclusive_scan(first, last, result, binary_op)
initialises its accumulator by moving out of the first element of the
input range:

  auto __init = _GLIBCXX_ITER_MOVE(__first);   // was: auto __init = *__first;

That happens unconditionally, so a plain out-of-place scan silently destroys
the caller's input. For a value type with a destructive move (e.g. std::string)
the first element is left empty.

A reduced testcase, which fails with g++ 16 and passes with g++ 15 in both
-std=c++17 and -std=c++20:

  #include <numeric>
  #include <string>
  #include <vector>
  #include <functional>
  #include <cassert>

  int main()
  {
    std::vector<std::string> in{"a", "b"}, out(2);
    std::inclusive_scan(in.begin(), in.end(), out.begin(),
                        std::plus<std::string>{});
    assert(out[1] == "ab");  // ok
    assert(in[0] == "a");    // fails: in[0] is an empty moved-from string
  }

Passing in.cbegin()/in.cend() instead leaves the input intact, so the
behaviour depends on the constness of the iterators handed in which is a
good sign it wasn't intended.

I believe it's non-conforming: [inclusive.scan]/6 specifies the effects
purely as assignments through result + K, and /9 permits result == first as
an aliasing allowance rather than as licence to modify the input when
result != first.

The move came in with gcc commit df1d436 ("libstdc++: Fix <numeric>
parallel algos for move-only values [PR117905]"), which relaxed the
requirement on the value type from CopyConstructible to MoveConstructible;
gcc 15 is unaffected. I've reported it upstream as GCC Bug 126604:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126604

Gentoo hit the same six failures with taskflow 3.10.0
(https://bugs.gentoo.org/961562) so updating to a newer upstream release won't
help.

This explains the failure set exactly: only the six InclusiveScan.string.*
cases fail, because the overloads taking an init value never touch the
input and the TransformInclusiveScan tests are int-only (a moved-from int
keeps its value). The test itself is correct: it computes its golden vector
with a perfectly ordinary out-of-place std::inclusive_scan and that call alone
is enough to corrupt the input, so no change to the test source can make it
pass legitimately while linked against libstdc++ 16.

So my plan to resolve this bug is to skip those six tests in debian/rules, via
ctest -E 'InclusiveScan\.string\.', with a comment pointing to the GCC bug,
and to file a separate Debian bug against taskflow to track enabling those tests
once libstdc++ 16 is fixed.

I guess I need to open a separate Debian bug on gcc-16 where I can link to
the upstream GCC bug report.

What do you think?


Cheers!

Christopher Obbard

debian-gcc@lists.debian.org was listed as recipient.

Members of debian-gcc@lists.debian.org: Please read on below.

Best,
Sven

We believe that the bug you reported is fixed in the latest version of
taskflow, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1133642@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Anton Gladky <gladk@debian.org> (supplier of updated taskflow package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Tue, 04 Aug 2026 20:28:37 +0200
Source: taskflow
Architecture: source
Version: 3.9.0+ds-2
Distribution: unstable
Urgency: medium
Maintainer: Debian Python Team <team+python@tracker.debian.org>
Changed-By: Anton Gladky <gladk@debian.org>
Closes: 1133642
Changes:
 taskflow (3.9.0+ds-2) unstable; urgency=medium
 .
   * Team upload.
   * [7721360] Add basic salsa-ci-yml
   * [370a211] Fix debian/copyright license mismatches found by licenserecon
   * [6610798] Fix data corruption in InclusiveScan with gcc-16/libstdc++-16.
               (Closes: #1133642)
   * [6aa691a] Make Doxygen search index generation reproducible
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