test errors: ambiguous overload for ‘operator=’, binding reference discards qualifiers

I'm trying to build xtensor 0.27.1 with xtl 0.8.1 (on a debian linux system).

Build-time tests (with g++) are passing, but runtime tests (tests subdir) using the installed packages are failing with both g++ 15.2.0 and clang 21.1.8.

g++ reports

[ 73%] Building CXX object CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o
/usr/bin/g++   -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c++20 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wno-sign-conversion  -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0 -O3 -D
NDEBUG -std=gnu++20 -MD -MT CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o -MF CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o.d -o CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o -c /tm
p/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp
...
In file included from /usr/include/xtensor/core/xsemantic.hpp:16,
                 from /usr/include/xtensor/views/xstrided_view.hpp:26,
                 from /usr/include/xtensor/core/xoperation.hpp:22,
                 from /usr/include/xtensor/core/xmath.hpp:28,
                 from /usr/include/xtensor/io/xio.hpp:22,
                 from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp:10:
/usr/include/xtensor/core/xassign.hpp: In instantiation of ‘void xt::stepper_assigner<E1, E2, L>::run() [with E1 = xt::xmasked_view<xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_t
ype::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&, xt::xarray_container<xt::uvector<bool, std::allocator<bool> >, xt::layout_type::row_major
, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&>; E2 = xt::xbroadcast<const xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layou
t_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true> >; xt::layo
ut_type L = xt::layout_type::row_major]’:
...
/usr/include/xtensor/views/xmasked_view.hpp:579:40:   required from ‘xt::xmasked_view<CTD, CTM>::self_type& xt::xmasked_view<CTD, CTM>::operator=(const xt::xexpression<E1>&) [with E = xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>; CTD = xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&; CTM = xt::xarray_container<xt::uvector<bool, std::allocator<bool> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&; self_type = xt::xmasked_view<xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&, xt::xarray_container<xt::uvector<bool, std::allocator<bool> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&>]’
  579 |         return semantic_base::operator=(e);
      |                ~~~~~~~~~~~~~~~~~~~~~~~~^~~
/tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp:221:23:   required from here
  221 |         masked_data = data2;
      |                       ^~~~~
/usr/include/xtensor/core/xassign.hpp:664:20: error: ambiguous overload for ‘operator=’ (operand types are ‘xt::xmasked_view_stepper<xt::xmasked_view<xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&, xt::xarray_container<xt::uvector<bool, std::allocator<bool> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&>, false>::reference’ {aka ‘xtl::xmasked_value<double&, bool&>’} and ‘xtl::xmasked_value<double&, bool&>’)
  664 |             *m_lhs = conditional_cast<needs_cast, result_type>(*m_rhs);
      |             ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/xtensor/core/xassign.hpp:664:20: note: there are 4 candidates
In file included from /usr/include/xtensor/views/xmasked_view.hpp:20,
                 from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp:12:
/usr/include/xtl/xmasked_value.hpp:92:9: note: candidate 1: ‘xtl::xmasked_value<T, B>& xtl::xmasked_value<T, B>::operator=(const T1&) [with T1 = xtl::xmasked_value<double&, bool&>; T = double&; B = bool&]’
   92 |         DEFINE_ASSIGN_OPERATOR(=);
      |         ^~~~~~~~~~~~~~~~~~~~~~
/usr/include/xtl/xmasked_value.hpp:92:9: note: candidate 2: ‘xtl::xmasked_value<T, B>& xtl::xmasked_value<T, B>::operator=(const xtl::xmasked_value<T1, B1>&) [with T1 = double&; B1 = bool&; T = double&; B = bool&]’
   92 |         DEFINE_ASSIGN_OPERATOR(=);
      |         ^~~~~~~~~~~~~~~~~~~~~~
/usr/include/xtl/xmasked_value.hpp:30:11: note: candidate 3: ‘xtl::xmasked_value<double&, bool&>& xtl::xmasked_value<double&, bool&>::operator=(const xtl::xmasked_value<double&, bool&>&)’ (deleted)
   30 |     class xmasked_value
      |           ^~~~~~~~~~~~~
/usr/include/xtl/xmasked_value.hpp:30:11: note: candidate 4: ‘xtl::xmasked_value<double&, bool&>& xtl::xmasked_value<double&, bool&>::operator=(xtl::xmasked_value<double&, bool&>&&)’ (deleted)
/usr/include/xtensor/core/xassign.hpp: In instantiation of ‘void xt::stepper_assigner<E1, E2, L>::run() [with E1 = xt::xmasked_view<xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_t
ype::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&, xt::xarray_container<xt::uvector<bool, std::allocator<bool> >, xt::layout_type::row_major
, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&>; E2 = xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt
::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>; xt::layout_type L = xt::layout_type::row_major]’:
/usr/include/xtensor/core/xassign.hpp:476:97:   required from ‘static void xt::xexpression_assigner_base<xt::xtensor_expression_tag>::assign_data(xt::xexpression<D>&, const xt::xexpression<E2>&, bool) [with E1 =
 xt::xmasked_view<xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression
_tag>&, xt::xarray_container<xt::uvector<bool, std::allocator<bool> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&>; E2 = xt::xarray_container<xt::uvector<double, std::allocator<double> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>]’
  476 |             stepper_assigner<E1, E2, default_assignable_layout(E1::static_layout)>(de1, de2).run();
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
...

Both g++ 15 and clang 21 report

/tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp:221:23:   required from here
  221 |         masked_data = data2;
      |                       ^~~~~
/usr/include/xtl/xmasked_value.hpp:123:11: error: binding reference of type ‘xtl::xmasked_value<double&, bool&>::value_type’ {aka ‘double&’} to ‘const double’ discards qualifiers
  123 |         : m_value(std::forward<T1>(value)), m_visible(true)
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/xtl/xmasked_value.hpp:123:55: error: cannot bind non-const lvalue reference of type ‘xtl::xmasked_value<double&, bool&>::flag_type’ {aka ‘bool&’} to an rvalue of type ‘bool’
  123 |         : m_value(std::forward<T1>(value)), m_visible(true)
      |                                                       ^~~~
make[3]: *** [CMakeFiles/test_xtensor_lib.dir/build.make:863: CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o] Error 1

That's from g++.
The message from clang 21 is similar:

[ 73%] Building CXX object CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o
/usr/bin/clang++   -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c++20 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wsign-conversion -Wold-style-cast -Wunused-variable -O3 -DNDEBUG -std=gnu++20 -MD -MT C
MakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o -MF CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o.d -o CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o -c /tmp/autopkgtest.bfBHFS/autopkgt
est_tmp/test_xmasked_view.cpp
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xio.cpp:24:
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_common_macros.hpp:10:
In file included from /usr/include/doctest/doctest.h:499:
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/ciso646:49:6: warning: "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros" [-W#warnings]
   49 | #    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
      |      ^
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xlayout.cpp:17:
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_common.hpp:19:
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_common_macros.hpp:10:
In file included from /usr/include/doctest/doctest.h:499:
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/ciso646:49:6: warning: "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros" [-W#warnings]
   49 | #    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
      |      ^
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmanipulation.cpp:18:
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_common_macros.hpp:10:
In file included from /usr/include/doctest/doctest.h:499:
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/ciso646:49:6: warning: "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros" [-W#warnings]
   49 | #    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
      |      ^
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp:14:
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_common_macros.hpp:10:
In file included from /usr/include/doctest/doctest.h:499:
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/ciso646:49:6: warning: "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros" [-W#warnings]
   49 | #    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
      |      ^
In file included from /tmp/autopkgtest.bfBHFS/autopkgtest_tmp/test_xmasked_view.cpp:12:
In file included from /usr/include/xtensor/views/xmasked_view.hpp:20:
/usr/include/xtl/xmasked_value.hpp:123:11: error: binding reference of type 'double' to value of type 'const double' drops 'const' qualifier
  123 |         : m_value(std::forward<T1>(value)), m_visible(true)
      |           ^       ~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/xtensor/io/../core/../core/../containers/../utils/../utils/xutils.hpp:682:20: note: in instantiation of function template specialization 'xtl::xmasked_value<double &, bool &>::xmasked_value<const double &>' requested here
  682 |             return static_cast<T>(std::forward<U>(u));
      |                    ^
/usr/include/xtensor/io/../core/../core/../containers/../utils/../utils/xutils.hpp:697:16: note: in instantiation of function template specialization 'xt::conditional_cast_functor<true, xtl::xmasked_value<double &, bool &>>::operator()<const double &>' requested here
  697 |         return conditional_cast_functor<condition, T>()(std::forward<U>(u));
      |                ^
/usr/include/xtensor/io/../core/../core/../views/../core/../core/xassign.hpp:664:22: note: in instantiation of function template specialization 'xt::conditional_cast<true, xtl::xmasked_value<double &, bool &>, const double &>' requested here
  664 |             *m_lhs = conditional_cast<needs_cast, result_type>(*m_rhs);
      |                      ^
...

The build-time tests (which pass) are compiling with

make  -f test/CMakeFiles/test_xmasked_view.dir/build.make test/CMakeFiles/test_xmasked_view.dir/depend
make[4]: Entering directory '/build/xtensor-0.27.1/obj-x86_64-linux-gnu'
cd /build/xtensor-0.27.1/obj-x86_64-linux-gnu && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /build/xtensor-0.27.1 /build/xtensor-0.27.1/test /build/xtensor-0.27.1/obj-x86_64-linux-gnu /build/xtensor-0.27.1/obj-x86_64-linux-gnu/test /build/xtensor-0.27.1/obj-x86_64-linux-gnu/test/CMakeFiles/test_xmasked_view.dir/DependInfo.cmake "--color=" test_xmasked_view
make[4]: Leaving directory '/build/xtensor-0.27.1/obj-x86_64-linux-gnu'
make  -f test/CMakeFiles/test_xmasked_view.dir/build.make test/CMakeFiles/test_xmasked_view.dir/build
make[4]: Entering directory '/build/xtensor-0.27.1/obj-x86_64-linux-gnu'
[ 48%] Building CXX object test/CMakeFiles/test_xmasked_view.dir/main.cpp.o
cd /build/xtensor-0.27.1/obj-x86_64-linux-gnu/test && /usr/bin/c++ -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS -I/build/xtensor-0.27.1/include -I/build/xtensor-0.27.1/obj-x86_64-linux-gnu -g -O2 -ffile-prefix-map=/build/xtensor-0.27.1=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2 -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c++20 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wno-sign-conversion  -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0 -std=gnu++20 -MD -MT test/CMakeFiles/test_xmasked_view.dir/main.cpp.o -MF CMakeFiles/test_xmasked_view.dir/main.cpp.o.d -o CMakeFiles/test_xmasked_view.dir/main.cpp.o -c /build/xtensor-0.27.1/test/main.cpp
..
[ 48%] Building CXX object test/CMakeFiles/test_xmasked_view.dir/test_xmasked_view.cpp.o
cd /build/xtensor-0.27.1/obj-x86_64-linux-gnu/test && /usr/bin/c++ -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS -I/build/xtensor-0.27.1/include -I/build/xtensor-0.27.1/obj-x86_64-linux-gnu -g -O2 -ffile-prefix-map=/build/xtensor-0.27.1=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2 -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c++20 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wno-sign-conversion  -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0 -std=gnu++20 -MD -MT test/CMakeFiles/test_xmasked_view.dir/test_xmasked_view.cpp.o -MF CMakeFiles/test_xmasked_view.dir/test_xmasked_view.cpp.o.d -o CMakeFiles/test_xmasked_view.dir/test_xmasked_view.cpp.o -c /build/xtensor-0.27.1/test/test_xmasked_view.cpp
...
[ 52%] Linking CXX executable test_xmasked_view
cd /build/xtensor-0.27.1/obj-x86_64-linux-gnu/test && /usr/bin/cmake -E cmake_link_script CMakeFiles/test_xmasked_view.dir/link.txt --verbose=1
/usr/bin/c++ -g -O2 -ffile-prefix-map=/build/xtensor-0.27.1=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2 -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c++20 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wno-sign-conversion  -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0 -Wl,-z,relro -Wl,--dependency-file=CMakeFiles/test_xmasked_view.dir/link.d CMakeFiles/test_xmasked_view.dir/main.cpp.o CMakeFiles/test_xmasked_view.dir/test_xmasked_view.cpp.o -o test_xmasked_view
make[4]: Leaving directory '/build/xtensor-0.27.1/obj-x86_64-linux-gnu'
[ 52%] Built target test_xmasked_view
..
[ 93%] Building CXX object test/CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o
cd /build/xtensor-0.27.1/obj-x86_64-linux-gnu/test && /usr/bin/c++ -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS -I/build/xtensor-0.27.1/include -I/build/xtensor-0.27.1/obj-x86_64-linux-gnu -g -O2 -ffile-prefi
x-map=/build/xtensor-0.27.1=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2 -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c+
+20 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wno-sign-conversion  -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0 -std=gnu++20 -MD -MT test/CMakeFiles/test_xtensor_lib.dir/test_xmasked_
view.cpp.o -MF CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o.d -o CMakeFiles/test_xtensor_lib.dir/test_xmasked_view.cpp.o -c /build/xtensor-0.27.1/test/test_xmasked_view.cpp

I'm not sure what the key difference between build-time and run-time tests is. I guess it's not the -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS flag.

The failing case compiled with -O3 -DNDEBUG instead of -g -O2 .

The passing case added hardening compilation flags -ffile-prefix-map=/build/xtensor-0.27.1=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2

Of those differences, -O3 instead of -O2 sounds most relevant to me. Could -O3 trigger "binding reference" errors?

Hmm, apparently the issue is -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS.

I can reproduce the error running the compile command from the command line. Then the otherwise failing compilation step completes successfully (with -O3) if I manually add -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS.

This build flag isn't being controlled explicitly in the debian build script. It's getting set by default by the toplevel CMakeLists.txt.

My (failing) use-case here is to run the tests as standalone tests, testing the installed xtensor library. So my workaround is to set -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS=ON manually when running these run-time tests.

Apart from that, what do you advise with respect to this build flag? Is xtensor expected to trigger binding reference errors if the flag not used?

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test errors: ambiguous overload for ‘operator=’, binding reference discards qualifiers #2887

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test errors: ambiguous overload for ‘operator=’, binding reference discards qualifiers #2887

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drew-parsons commented on Feb 9, 2026

drew-parsons commented on Feb 9, 2026

drew-parsons commented on Feb 9, 2026

drew-parsons commented on Feb 9, 2026

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