[cfe-dev] C++17 Question | Proposal N3922

[James Dennett via cfe-dev]

On Tue, Jan 9, 2018 at 2:52 AM, José Luis March Cabrelles via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> my name is José Luis, I have a question regarding backwards compatibility
> of proposal N3922 for C++17:
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3922.html
>

There's more information available in the papers referenced by n3922: "For
background information see N3681
<http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3681.html>, "Auto and
braced-init-lists", by Voutilainen, and N3912, "Auto and braced-init-lists,
continued", also by Voutilainen."

>
>
> That change introduces a new incompatibility with C++14 and C++11, as it
> is stated in the C++17 Standard:
> C.4.3 Clause 10: declarations [diff.cpp14.dcl.dcl]
> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2038
>
> Is it considered a New Feature or a Defect Report?
>

Quoting from n3922: "Direction from EWG is that we consider this a defect
in C++14."


>
> I am asking this because Clang (and GCC) adopted the new rule not only for
> C++17 but also for C++14 and C++11. In fact, I found this comment:
> This is a backwards-incompatible change that is applied to all language
> versions that allow type deduction from auto (per the request of the C++
> committee).
> http://clang.llvm.org/cxx_status.html#n3922
>
> Can someone please confirm and explain why the C++ committee requested
> compiler developers to treat this new rule as a Defect Report and, hence,
> to fix/change it also in C++11 and C++14?
>

Ville Voutilainen's n3912.html gives more information on that.

n3681.html mentions the example of init-captures.  Using the original
wording of C++14, `[a{b}]` would not have deduced `a` as a copy of `b`,
although it was the intent of "uniform initialization" that it should do
so.  When the rules as written fail to capture the intent in basic cases,
the committee might consider it a defect, as in this case.

-- James
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