[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

["rodgertq at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

--- Comment #6 from Thomas Rodgers <rodgertq at gcc dot gnu.org> ---
It does raise an issue which I'm going to follow up on separately on the SG1
(concurrency and parallelism study group) mailing list.

While it is indeed the case that standard says you can't count on deterministic
sequencing for std::reduce(), I can't find any wording that directly supports
the wording on cppreference.com std::execution::sequenced_policy, though that
wording is consistent with my recollection of SG1's discussions regarding
execution policies.

There is wording that says 'Unless otherwise specified, the semantics of
ExecutionPolicy algorithm overloads are identical to their overloads without."

The wording for std::for_each() for the non-execution policy overload the
standard says -

Effects: Applies f to the result of dereferencing every iterator in the range
[first, last), starting from first and proceeding to last - 1.

And for one that takes an execution policy -

Effects: Applies f to the result of dereferencing every iterator in the range
[first, last).

So it omits the ordering constraint, which makes sense, but I wonder if we
shouldn't be more explicit in the documentation of the policies themselves.