[Bug libstdc++/99522] New: Using the unseq execution policy in min_element requires the sequence value type to be copy assignable

[Bug libstdc++/99522] New: Using the unseq execution policy in min_element requires the sequence value type to be copy assignable

[pilarlatiesa at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99522

            Bug ID: 99522
           Summary: Using the unseq execution policy in min_element
                    requires the sequence value type to be copy assignable
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pilarlatiesa at gmail dot com
  Target Milestone: ---

Many algorithms have the precondition that *first meets the Cpp17MoveAssignable
requirements. However, I don’t see such a requirement for min_element,
max_element, or minmax_element. In spite of that, the following snippet is
rejected:
$ cat test.cpp
#include <vector>
#include <algorithm>
#include <execution>

struct T
{
  int const ID; // Not Cpp17CopyAssignable
};

int f(std::vector<T> const &v)
{
  if (v.empty()) return -1;

  return std::min_element(std::execution::par_unseq, v.begin(), v.end(),
                          [](T const &lhs, T const &rhs) { return lhs.ID <
rhs.ID; })->ID;
}

The error message is:
$ g++-10 -std=c++17 -c test.cpp
[…]
error: use of deleted function 'T& T::operator=(const T&)'
  643 |                 __min_val = __obj.__min_val;
      |                 ~~~~~~~~~~^~~~~~~~~~~~~~~~~
[…]

There’s also
[algorithms.parallel.exec]/3 Unless otherwise stated, implementations may make
arbitrary copies of elements (with type T) from sequences where
is_trivially_copy_constructible_v<T> and is_trivially_destructible_v<T> are
true.
but I’ve been told that it grants copy construction, but not copy assignment.

$ g++-10 -v
Using built-in specs.
COLLECT_GCC=g++-10
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
10.2.0-5ubuntu1~20.04' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-10-WJNXnb/gcc-10-10.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-WJNXnb/gcc-10-10.2.0/debian/tmp-gcn/usr,hsa
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (Ubuntu 10.2.0-5ubuntu1~20.04)