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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99117] [9/10/11 Regression] cannot accumulate
 std::valarray
Date: Tue, 23 Feb 2021 13:56:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99117

--- Comment #15 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So, based on IRC discussion, the problem is that in:
  template<typename _Tp> template<class _Dom>
    inline valarray<_Tp>&
    valarray<_Tp>::operator=3D(const _Expr<_Dom, _Tp>& __e)
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 630. arrays of valarray.
      if (_M_size =3D=3D __e.size())
        std::__valarray_copy(__e, _M_size, _Array<_Tp>(_M_data));

the temporary _Array<_Tp>(_M_data) object's _M_data passed to __valarray_co=
py
doesn't fully own the object and that __e's operator[] aliases with that.
Can't libstdc++ for cases where such aliasing is possible use an _Array-like
class without the __restrict (i.e. something that wouldn't lie to the compi=
ler
that it fully owns it)?=