[cfe-dev] [libc++] c++14 return type deduction usage should be limited

[Marshall Clow]

On Aug 6, 2013, at 3:39 PM, Zhihao Yuan <lichray at gmail.com> wrote:

> Hi, hackers:
> 
> I noticed that in include/functional:
> 
>   _LIBCPP_INLINE_VISIBILITY auto operator()(_T1&& __t, _T2&& __u) const
> 
> is widely used to support C++14 type deduced function objects:
> 
>  http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3421.htm
> 
> However, in the standard text, the return type is specified with
> 
>  -> decltype(std::forward<T>(t) + std::forward<U>(u));
> 
> There is a slight difference between using return type deduction and
> writing `decltype` out explicitly, because the later case triggers
> SFINAE, while the first case does not.  See:
> 
>  http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3638.html
> 
>   "SFINAE
> 
>    Since the return type is deduced by instantiating the template, if the
>    instantiation is ill-formed, this causes an error rather than a
>    substitution failure. [...]"
> 
> I suggest to obey the standard here, even such a behavior may
> (arguably) not be intentional.
> 
> BTW, in the adopted text, each of the new type deduced function
> class has a `is_transparent` typedef, so…

Well, crud.
I did that - and I left those bits off because I convinced myself that the two formulations were the same.

I can apply your patch if you would like - but do you have any ideas for writing test cases?

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

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