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### 2822. Resolution for LWG 2742 introduces ambiguities

Section: 23.4.3.3 [string.cons] Status: NAD Submitter: Jonathan Wakely Opened: 2016-11-15 Last modified: 2017-01-30

Priority: Not Prioritized

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Discussion:

The new constructor added for LWG 2742 causes an ambiguity when attempting to construct from a (non-const) `char*`

>     char s[] = "whoops";
>     std::string str(s, 2, 4);

This is the same problem discussed in 2758, and the fix is the same: disable that new constructor if `T` is convertible to `const_pointer`, so that the old constructor is used instead.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify the new `basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());` constructor in 23.4.3.3 [string.cons] as shown:

>    >     template<class T>
>    >     basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());

>    > > -?- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then behaves the same as:

>    > > >     basic_string(sv.substr(pos, n), a)

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

[2016-12-21, Jonathan comments and reommends NAD]

Tim Song pointed out that this constructor isn't ambiguous, and I can no longer reproduce the errors I was getting, so I think this issue should be resolved NAD. The code snippet in the issue discussion compiles OK without the proposed change.

[2017-01-27 Telecon]

NAD

Proposed resolution: