[PATCH] PR libstdc++/79162 ambiguity in string assignment due to string_view overload (LWG 2946)

[Daniel Krügler]

2017-07-28 22:40 GMT+02:00 Daniel Krügler <daniel.kruegler@gmail.com>:
> 2017-07-28 22:29 GMT+02:00 Daniel Krügler <daniel.kruegler@gmail.com>:
>> 2017-07-28 22:25 GMT+02:00 Tim Song <t.canens.cpp@gmail.com>:
>>> On Fri, Jul 28, 2017 at 4:10 PM, Daniel Krügler
>>> <daniel.kruegler@gmail.com> wrote:
>>>> +      // Performs an implicit conversion from _Tp to __sv_type.
>>>> +      template<typename _Tp>
>>>> +        static __sv_type _S_to_string_view(const _Tp& __svt)
>>>> +        {
>>>> +          return __svt;
>>>> +        }
>>>
>>> I might have gone for
>>>
>>> +        static __sv_type _S_to_string_view(__sv_type __svt) noexcept
>>> +        {
>>> +          return __svt;
>>> +        }
>>>
>>> With that, we can also use noexcept(_S_to_string_view(__t)) to make up
>>> for the absence of is_nothrow_convertible (basically the same thing I
>>> did in LWG 2993's PR).
>>
>> Agreed, that makes very much sense. I will adjust the P/R, but before
>> I resubmit I would like to get feedback whether the other two compare
>> functions also should become conditionally noexcept.
>
> Locally I have now performed the sole change of the _S_to_string_view
> declaration getting rid of the template, but would also like to gather
> feedback from the maintainers whether I should also change the form of
> the conditional noexcept to use the expression
>
> noexcept(_S_to_string_view(__t))
>
> instead of the current
>
> is_same<_Tp, __sv_type>::value
>
> as suggested by Tim Song.
>
> I'm asking also, because I have a paper proposing to standardize
> is_nothrow_convertible submitted for the upcoming C++ mailing - This
> would be one of the first applications in the library ;-)

A slightly revised patch update: It replaces the _S_to_string_view
template by a simpler _S_to_string_view function as of Tim Song's
suggestion, but still uses the simplified noexcept specification
deferring it to a future application case for is_nothrow_convertible.
Furthermore now all three compare function templates are now
(conditionally) noexcept by an (off-list) suggestion from Jonathan
Wakely.

Thanks,

- Daniel
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