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From: "rodgertq at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/101274] [11/12 Regression] std::execution::seq has
 incorrect behaviour under GCC 11.1.0
Date: Fri, 02 Jul 2021 01:03:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101274

--- Comment #4 from Thomas Rodgers <rodgertq at gcc dot gnu.org> ---
I did some more reading of
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4878.pdf and it wo=
uld
seem that you are not even guaranteed a deterministic ordering of the
application of binary_op on the non-execution policy accepting overloads of
std::reduce().

See [reduce] the note at bullet 9 -

"[Note 1 : The difference between reduce and accumulate is that reduce appl=
ies
binary_op in an unspecified order, which yields a nondeterministic result f=
or
non-associative or non-commutative binary_op such as floating-point additio=
n. =E2=80=94
end note]"=