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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/101542] __gnu_cxx::sequence_buffer const copy
 constructor is badly broken
Date: Wed, 21 Jul 2021 14:24:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101542

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Brooks Moses from comment #0)
> This started failing with a recent Clang change
> (https://github.com/llvm/llvm-project/commit/
> 7d2d5a3a6d7aaa40468c30250bf6b0938ef02c08), described as "Apply P1825 as
> Defect Report from C++11 up to C++20".  See
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1825r0.html for =
the
> defect report details.  I would guess that GCC will be applying a similar
> change.

GCC 11 already implemented that, see PR 91427 and the commit
https://gcc.gnu.org/g:1722e2013f05f1f1f99379dbaa0c0df356da731f

The for that commit says:

    Discussion on the CWG reflector about how to avoid breaking the PR91212
test
    in the new model settled on the model of doing only a single overload
    resolution, with the variable treated as an xvalue that can bind to
    non-const lvalue references.  So this patch implements that approach.  =
The
    implementation does not use the existing LOOKUP_PREFER_RVALUE flag, but
    instead sets a flag on the representation of the static_cast turning the
    variable into an xvalue.

which says that calling sequence_buffer(sequence_buffer&) here is intended,
which is why we didn't see any change in the ext/rope/4.cc test when GCC
implemented it.

I still think we want to make sequence_buffer move-aware, so that you get t=
he
same behaviour for:

template<typename T> T f(T x) { return x; }
template<typename T> T g(T x) { return std::move(x); }

when passed a sequence_buffer.=