https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274
--- 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 would 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 applies binary_op in an unspecified order, which yields a nondeterministic result for non-associative or non-commutative binary_op such as floating-point addition. — end note]"
