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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/107572] cartesian_product_view invokes the begin of
 input_range twice
Date: Thu, 09 Mar 2023 18:25:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107572

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:3df9760d56662bdf38dd45f7398f003bbd64fdfe

commit r13-6558-g3df9760d56662bdf38dd45f7398f003bbd64fdfe
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Mar 9 13:25:44 2023 -0500

    libstdc++: extraneous begin in cartesian_product_view::end [PR107572]

    ranges::begin() isn't guaranteed to be equality-preserving for non-forw=
ard
    ranges, so in cartesian_product_view::end we need to avoid needlessly
    calling begin() on the first range (which could be non-forward) in the
    case where __empty_tail is false as per its specification.

    Since we're already using a variadic lambda to compute __empty_tail, we
    might as well use that same lambda to build up the tuple of iterators
    instead of building it separately via e.g. std::apply or __tuple_transf=
orm.

            PR libstdc++/107572

    libstdc++-v3/ChangeLog:

            * include/std/ranges (cartesian_product_view::end): When
            building the tuple of iterators, avoid calling ranges::begin on
            the first range if __empty_tail is false.
            * testsuite/std/ranges/cartesian_product/1.cc (test07): New tes=
t.=