| Doc. no. | N4539 |
| Date: | 2015-05-22 |
| Project: | Programming Language C++ |
| Reply to: | Ville Voutilainen <ville.voutilainen@gmail.com> |
Revised 2015-05-22 at 17:05:15 UTC
Reference ISO/IEC IS 14882:2003(E)
Also see:
The purpose of this document is to record the status of issues which have come before the Evolution Working Group (EWG) of the INCITS PL22.16 and ISO WG21 C++ Standards Committee. Issues represent potential defects in the ISO/IEC IS 14882:2003(E) document, and proposed extensions to it.
This document contains only evolution issues which are actively being considered by the Evolution Working Group, i.e., issues which have a status of New, Open, Ready, or Review. See Evolution Completed Issues List for issues considered completed (adopted) and Evolution Closed Issues List for issues considered closed (rejected).
The issues in these lists are not necessarily formal ISO Defect Reports (DR's). While some issues will eventually be elevated to official Defect Report status, other issues will be disposed of in other ways. See Issue Status.
This document includes [bracketed italicized notes] as a reminder to the EWG of current progress on issues. Such notes are strictly unofficial and should be read with caution as they may be incomplete or incorrect. Be aware that EWG support for a particular resolution can quickly change if new viewpoints or killer examples are presented in subsequent discussions.
For the most current official version of this document see http://www.open-std.org/jtc1/sc22/wg21/. Requests for further information about this document should include the document number above, reference ISO/IEC 14882:2003(E), and be submitted to Information Technology Industry Council (ITI), 1250 Eye Street NW, Washington, DC 20005.
Public information as to how to obtain a copy of the C++ Standard, join the standards committee, submit an issue, or comment on an issue can be found in the comp.std.c++ FAQ.
New - The issue has not yet been reviewed by the EWG. Any Wording available is purely a suggestion from the issue submitter, and should not be construed as the view of EWG.
Open - The EWG has discussed the issue but is not yet ready to move the issue forward. There are several possible reasons for open status:
A Wording available for an open issue is still not be construed as the view of EWG. Comments on the current state of discussions are often given at the end of open issues in an italic font. Such comments are for information only and should not be given undue importance.
Deferred - The EWG has discussed the issue, is not yet ready to move the issue forward, but neither does it deem the issue significant enough to delay publishing a standard or Technical Report. A typical deferred issue would be seeking to clarify wording that might be technically correct, but easily mis-read.
A Wording available for a deferred issue is still not be construed as the view of EWG. Comments on the current state of discussions are often given at the end of open issues in an italic font. Such comments are for information only and should not be given undue importance.
Dup - The EWG has reached consensus that the issue is a duplicate of another issue, and will not be further dealt with. A Rationale identifies the duplicated issue's issue number.
NAD - The EWG has reached consensus that the issue is not a defect in the Standard nor is it an extension the EWG deems acceptable.
Review - Exact resolution is now available for review on an issue for which the EWG previously reached informal consensus.
Ready - The EWG has reached consensus that the issue is an extension that can go forward to Core, Library, or a Study Group for further processing.
Resolved - The EWG has reached consensus that the issue is a defect in or an acceptable extension to the Standard, but the resolution adopted to resolve the issue came via some other mechanism than this issue in the list - typically by applying a formal paper, occasionally as a side effect of consolidating several interacting issue resolutions into a single issue.
DR - (Defect Report) - It's not expected that the EWG would handle Defect Reports.
WP - (Working Paper) - The proposed resolution has not been accepted as a Technical Corrigendum, but the full WG21/PL22.16 committee has voted to apply the issue's resolution to the working paper.
Tentatively - This is a status qualifier. The issue has been reviewed online, or at an unofficial meeting, but not in an official meeting, and some support has been formed for the qualified status. Tentatively qualified issues may be moved to the unqualified status and forwarded to full committee (if Ready) within the same meeting. Unlike Ready issues, Tentatively Ready issues will be reviewed in subcommittee prior to forwarding to full committee. When a status is qualified with Tentatively, the issue is still considered active.
Pending - This is a status qualifier. When prepended to a status this indicates the issue has been processed by the committee, and a decision has been made to move the issue to the associated unqualified status. However for logistical reasons the indicated outcome of the issue has not yet appeared in the latest working paper.
Issues are always given the status of New when they first appear on the issues list. They may progress to Open or Review while the EWG is actively working on them. When the EWG has reached consensus on the disposition of an issue, the status will then change to Dup, NAD, or Ready as appropriate. Once the full J16 committee votes to forward Ready issues to the Project Editor, they are given the status of Defect Report ( DR). These in turn may become the basis for Technical Corrigenda (TC1), or are closed without action other than a Record of Response (Resolved ). The intent of this EWG process is that issues which are defects in or accepted extensions to the Standard move to the formal ISO DR status.
Section: 4.13 [conv.rank] Status: Open Submitter: Jens Maurer Opened: 2012-09-12 Last modified: 2015-05-22
View all issues with Open status.
Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3387.html
Deemed post-C++14 material in Chicago 2013.
Discussed in Rapperswil 2014. EWG stated that in order to move forward with such a proposal, it would need to provide data about the compatibility/breakage issues, if any.
Wording available:
The paper contains the proposed wording.
Section: 18.6 [support.dynamic] Status: Open Submitter: Clark Nelson Opened: 2012-08-30 Last modified: 2015-05-22
View all issues with Open status.
Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3396.htm
Reviewed by EWG in Portland, author encouraged to revise.
Deemed post-C++14 material in Chicago 2013. Has an associated NB comment, FI 16, although the comment is rejected for C++14.
Wording available:
The paper contains the proposed wording that is to be revised.
Section: 16.3 [cpp.replace] Status: New Submitter: Jonathan de Boyne Pollard Opened: 2012-09-21 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3400.htmlWording available:
The paper contains the proposed wording.
Section: 14 [temp] Status: Open Submitter: Mike Spertus Opened: 2012-09-22 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4469.html
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3405.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3601.html
EWG review started, not completed yet. Likely needs a follow-up paper.
Bristol 2013: Encouraged to pursue further. Template parameter deduction for constructors has been split into EWG Issue 60.
Discussed in Lenexa. The author was encouraged to strive for a syntax that uses auto. The author will revise and come back.
Section: 17 [library] Status: New Submitter: Dietmar Kühl Opened: 2012-09-14 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3407.htmlHandled by the Numerics Study Group (SG6).
Section: 1.10 [intro.multithread] Status: New Submitter: Pablo Halpern Opened: 2012-09-24 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3409.pdfHandled by the Concurrency Study Group (SG1)
Section: 3.9 [basic.types] Status: Open Submitter: Jens Maurer Opened: 2012-09-19 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3412.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3467.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3497.html
Reviewed by EWG in Portland 2012, proceeding to CWG. The library part is N2648 C++ Dynamic Arrays, and that part is proceeding to LWG.
Accepted into the Working Draft in Bristol 2013, as N3639.
Moved in Chicago 2013 to be designated for an Array Extension TS, which hasn't materialized yet.
Wording available:
The paper contains the proposed wording, as does the Library counterpart.Section: 14.1 [temp.param] Status: Open Submitter: Jens Maurer Opened: 2012-09-19 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3413.html
Bristol 2013: Maurer expressed surprise at the paper being under discussion, and explained that he doesn't think it can be made to work under current linker environments, and further explained that user-defined equality operators cause confusion and surprises. Maurer said that he'd want Stroustrup to clarify which parts of the paper he would want.
Two-way Straw polls:
Rules for agument expressions:
F: 5 A: 0
Structs without operator==
F: 0 A: 0
Structs with operator==
F: 1 A: 3
The issue is not pushed at this time.
Deemed post-C++14 material in Chicago 2013, Stroustrup expressed interest in writing papers about the subject targeting C++17.
Wording available:
The paper contains the proposed wording.Section: 14.1 [temp.param] Status: New Submitter: Mike Spertus Opened: 2012-09-21 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3416.html
There is a closed (extension status) Core issue for this, see Core issue 1643.
Section: 1.10 [intro.multithread] Status: Open Submitter: Robert Geva Opened: 2012-09-21 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3419.pdfHandled by the Concurrency Study Group (SG1).
Section: 30 [thread] Status: Open Submitter: Artur Laksberg Opened: 2012-09-21 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3429.pdfHandled by the Concurrency Study Group (SG1).
Wording available:
The paper contains the proposed wording.
Section: 18.1 [support.general] Status: Open Submitter: Clark Nelson Opened: 2012-09-18 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4030.htm
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3745.htm
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3694.htm
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3435.htm
Reviewed by EWG in Portland 2012, proceeding in SG10, Feature Test.
Handled by SG10 to the first desired outcome in Chicago2013, no need for further EWG follow-up.
Discussed again in Rapperswil 2014.
Straw poll, has_cpp_attribute as a function-style macro:
SF 7, WF 9, N 4, WA 0, SA 0.
EWG's guidance was for the function-style macro.
Section: 20.9 [meta] Status: Open Submitter: Axel Naumann Opened: 2012-09-24 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3437.pdfNot reviewed by EWG yet, to be handled by the Reflection Study Group (SG7).
In Chicago 2013, EWG decided to let SG7 handle this.
Section: 18.8 [support.exception] Status: New Submitter: Aurelian Melinte Opened: 2012-09-20 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3441.htmlSection: 8.3.5 [dcl.fct] Status: New Submitter: Lawrence Crowl Opened: 2012-09-23 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3445.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3538.html
Deemed post-C++14 material in Chicago 2013.
Section: 5.2.7 [expr.dynamic.cast] Status: New Submitter: Bjarne Stroustrup Opened: 2012-09-23 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3449.pdf
Deemed post-C++14 material in Chicago 2013.
Section: 20.9 [meta] Status: Open Submitter: Herb Sutter, Ville Voutilainen Opened: 2012-01-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4461.html
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3329.pdf
Reviewed by EWG in Portland 2012, deemed to be handled by the Concepts Study Group (SG8).
Deemed post-C++14 material in Chicago 2013. SG8 isn't including it in their scope for the near future. Voutilainen is planning to write a simplified proposal for C++17.
Discussed in Lenexa. EWG encouraged the author to come back in Kona with a more detailed proposal.
Section: 14.5.3 [temp.variadic] Status: Open Submitter: Dave Abrahams Opened: 2012-10-16 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4235.htm
There are lots of very basic manipulations that are either really hard or impossible to do with argument packs unless you use something that causes a big recursive template instantiation, which is expensive at compile-time and can cause bad error messages. I want to be able to index argument packs with integral constant expressions, "take" or "drop" the first N elements of the pack, etc.
In Bristol 2013: N3493 may solve parts of the problem. The submitter is encouraged to write a paper, and practical examples are desirable.
N3761 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3761.html seems related.
Discussed in Rapperswil 2014. Vandevoorde expressed desire to write a paper.
The work done by George Makrydakis at https://github.com/irrequietus/atpp is related.
Discussed in Urbana. The author is encouraged to revise.
Section: 17.6.3.4 [hash.requirements] Status: Open Submitter: Matt Austern Opened: 2012-10-23 Last modified: 2015-05-22
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Discussion:
We have a hash function for built-in types and for some standard library types, but we don't have automatically generated hash<> specializations for user-defined types like
struct my_type {
int x;
std::string y;
vector<int> z;
};
Defining a good and efficient hash function for composite types takes a fair amount of work. One consequence is that there are a lot of user-defined types with bad hash functions floating around.
One possibility is automatically generating hash<> specializations, but that's tricky. A simpler possibility is providing tools that make it easier for users to do the right thing.
Bristol 2013: Austern explained that he didn't envision syntax to automate the generation of hash operations but thought that this could potentially be solved by a library. Stroustrup and Austern thought that reflection would be another way to solve this. Van Winkel thought that for the generation of such things, it's perhaps desirable that they aren't generated by default but can be generated on demand when a user-defined type requests such generation. The guidance of the EWG is to propose a solution that handles equality operators and other such things in a more general manner.
EWG expressed long-term interest in this idea in Chicago 2013 for post-C++14. Papers welcome.
Section: 3.4 [basic.lookup] Status: Open Submitter: Jeffrey Yasskin Opened: 2012-10-24 Last modified: 2015-05-22
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Discussion:
Sometimes we want to define a variable that's unused except for its constructor and destructor. lock_guard<mutex> and ScopeGuard are decent examples of this. In C++11, we have to manually name the variable something unique. Sometimes we use _some_name_##__LINE__ (suitably wrapped so the concatenation happens after expanding __LINE__) to try to generate unique names automatically, and gcc/clang have an extension _some_name_##__COUNTER__
http://gcc.gnu.org/onlinedocs/gcc-4.7.2/cpp/Common-Predefined-Macros.html
to allow multiple such variables on the same line. These are pretty verbose and not convenient for casual use. Haskell allows _ (underscore) to stand in for a variable that's not going to be used. Googlemock defines testing::_ to mean "don't care" as an argument, which is similar but not identical.Bristol 2013: Stroustrup wondered how unique the name needs to be, and wondered whether parallel builds would have problems ensuring the uniqueness. Naumann pointed out that having an unnamed variable is useful also for cases where you don't want the variable's address to be taken etc. Stroustrup and Van Winkel said this is not tiny, and a proper paper is necessary for this issue.
Chicago 2013: Deemed not as C++14 material, Yasskin or someone else invited to write a paper, straw polls in favor of the feature. Things to consider in the paper: Consider double underscore "__". Can it be used only in local scope? For class members? For globals?
Discussed in Rapperswil 2014. Still encouraging a paper, Dennett to contact Yasskin about it.
Section: 14.7.3 [temp.expl.spec] Status: Open Submitter: Faisal Vali Opened: 2012-10-27 Last modified: 2015-05-22
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Discussion:
I had submitted a DR (727) about this in October 2008 - and it was classified as an extension - I wonder if Spertus' DR (1077) that was also classified as an extension should be considered along with this one. 14.7.3 [temp.expl.spec] paragraph 2 requires that explicit specializations of member templates be declared in namespace scope, not in the class definition. This restriction does not apply to partial specializations of member templates; that is,
struct A {
template<class T> struct B;
template <class T> struct B<T*> { }; // well-formed
template <> struct B<int*> { }; // ill-formed
};
There does not seem to be a good reason for this inconsistency.
Bristol 2013: Defer to Core, with the guidance to reopen the DR mentioned and remove the restriction.
Before this can go over to Core, it needs wording. It's likely that it needs a paper. Vali should create either the wording or the paper.
Section: 6.5.4 [stmt.ranged] Status: Open Submitter: Gabriel Dos Reis Opened: 2013-01-12 Last modified: 2015-05-22
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Discussion:
The new-style 'for' syntax allows us to dispense with administrative iterator declarations when iterating over a single seuqence. The burden and noise remain, however, when iterating over two or more sequences simultaenously. We should extend the syntax to allow that. E.g. one should be able to write:
for (auto& x : v; auto& y : w)
a = combine(v, w, a);
instead of the noisier
auto p1 = v.begin();
auto q1 = v.end();
auto p2 = w.begin();
auto q2 = w.end();
while (p1 < q1 and p2 < q2) {
a = combine(*p1, *p2, a);
++p1;
++p2;
}
Bristol 2013: Submitter is encouraged to write a paper.
EWG expressed reiterated interest in Chicago 2013 for this idea, deeming it post-C++14 material.
Discussed in Rapperswil 2014. The author is still encouraged to submit a paper. Vandevoorde to contact Dos Reis and Lavavej about it.
Section: 14.7.3 [temp.expl.spec] Status: Open Submitter: Mike Spertus Opened: 2013-03-06 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3867.html
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3730.html
There is a closed (extension status) Core issue for this, see Core issue 1077.
This is a proposal to allow specializing templates from within a different namespace. The motivation is that when we declare a new class, it is natural to want to provide associated template specializations. For example, it is really painful that whenever I declare a class, I need to leave my namespace and enter namespace std just to specialize std::less as shown below
namespace A {
namespace B {
class C {...};
}
}
namespace std {
template <>
struct less<C> : binary_function <C, C, bool> {
bool operator() (const C & x, const C & y) const {...}
};
}
namespace A {
namespace B {
... // Continue working in A::B
}
}
Instead, I should be able to specialize std::less without having to break out of my namespace:
namespace A {
namespace B {
class C {...};
template <>
struct ::std::less<C> : binary_function <C, C, bool> {
bool operator() (const C & x, const C & y) const {...}
};
... // Continue working in A::B
}
}
Bristol 2013: Stroustrup expressed concern about unqualified name lookup in the specializations, and Voutilainen thought that that just might be the reason why the current rules don't allow it. Gottschling voiced concern about the implementation impact, and Voutilainen suggested asking for a quick review of the overall idea from Spicer. Austern thought that this could be palatable if it's expressed as a set of rewrite rules. Spertus asked about an alternative which is to be able to open another namespace without having to exit the current namespace. This alternative didn't gain success. Spertus to write a paper.
In Chicago 2013, EWG guidance was to work based on the current proposal N3730 without the facility of specializing a namespace-scoped template from inside a class.
Discussed in Rapperswil 2014.
Gregor and Vandevoorde expressed concern about changing name lookup, and thought that the problem space needs the help of Core experts. Dos Reis expressed concern about expanding the need to do more tentative parsing to resolve ambiguities. Stroustrup requested further study in order to explore the possibilities to find a solution that doesn't cause issues with lookup, and Spertus agreed that further exploration seems wise.
EWG encouraged working further on the problem, looking at alternative solutions.
Section: 2.2 [lex.phases] Status: New Submitter: Beman Dawes Opened: 2012-11-02 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3463.htmlSection: 18.1 [support.general] Status: New Submitter: Mike Spertus Opened: 2012-11-03 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3466.htmlSection: 3.4.2 [basic.lookup.argdep] Status: New Submitter: Dave Abrahams Opened: 2012-10-31 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3490.htmlSection: 7.1.3 [dcl.typedef] Status: Open Submitter: Walter Brown Opened: 2013-01-11 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3741.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3515.pdf
Reviewed in Chicago 2013, author encouraged to pursue the idea further with revised papers.
Section: 5.19 [expr.const] Status: Open Submitter: Scott Schurr Opened: 2013-03-13 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3583.pdf
Bristol 2013: We won't move forward with this at this time, but we might want to see a followup paper focusing on the trait.
Section: 6.5 [stmt.iter] Status: Open Submitter: Alan Talbot Opened: 2013-03-17 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3587.pdf
Bristol 2013: Van Winkel pointed out that this allows jumping out of nested loops. Naumann expressed doubt over whether the problem is so big that we need such a big extension to solve it. Van Winkel and Spertus thought that this would likely be a popular feature. Voutilainen thought that it would be beneficial to revisit a lambda solution. Talbot expressed doubt whether that's a suitable solution, syntax-wise and performance-wise. Austern thought that this seems to be in flux, and thought we aren't necessarily ready to choose between the various options. Gottschling thought Vandevoorde's option is nice, since it's still structured. Spertus said he likes the idea of having a control structure be an expression. Austern recommended looking closely at Clause 5 in the follow-up paper.
The author is encouraged to write a follow-up paper.
Section: 3.4.2 [basic.lookup.argdep] Status: Open Submitter: Peter Gottschling Opened: 2013-03-15 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3595.pdf
Bristol 2013: Looked at briefly, the EWG thinks this should be considered alongside other ADL proposals.
Section: 3.4.2 [basic.lookup.argdep] Status: New Submitter: Peter Gottschling Opened: 2013-03-15 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3596.pdf
Section: 14.8.2 [temp.deduct] Status: Open Submitter: Mike Spertus Opened: 2013-03-14 Last modified: 2015-05-22
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Discussion:
This issue is split from EWG Issue 9.
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4471.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3602.html
Bristol 2013: Reviewed and accepted by EWG, needs redrafting before ready for core.
Core pointed out problems in Bristol. Gregor summarized in Chicago 2013 that The primary template might not be the right place to pick constructors from. (Partial) specializations might have completely different constructors. Stroustrup thought that there's only two ways: all specializations have to be in scope, and you look at all of those, or look only at the primary and give an error if that doesn't work. Spertus is encouraged to write a follow-up paper.
Discussed in Lenexa. EWG gave guidance on the "typed constructors" technique, and encouraged the author to revise and come back.
Section: 20.8.2 [func.require] Status: New Submitter: Philipp Juschka Opened: 2013-03-14 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3617.htm
Section: 2.14.8 [lex.ext] Status: Open Submitter: Richard Smith Opened: 2013-03-13 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3599.html
Bristol 2013:
Straw Poll: Adopt N3599, send to core: SF:2 WF:1 N:6 WA:4 SA:1
No consensus for moving forward as is.
Straw Poll: Revise with additional machinery for compile time string processing
SF: 10 WF: 2 N: 0 WA: 0 SA: 0
Encouragement for Smith and Vandevoorde to revise.
Section: 6.4.2 [stmt.switch] Status: Open Submitter: Zhihao Yuan Opened: 2013-02-02 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3627.html
Reviewed in Chicago 2013, author is encouraged to pursue this further.
Section: 8.3.1 [dcl.ptr] Status: Ready Submitter: Hal Finker, Hubert Tong, M. Wong, R. Silvera, R. Mak, C. Cambly, et al. Opened: 2013-04-29 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4150.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3988.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3635.pdf
See also issue 26.
According to Wong, this paper was reviewed on the last day in EWG in Chicago 2013, although with no recorder. The summary was that many people liked the 1st solution (AliasGrouping), and authors are encouraged to develop it further.
Discussed further in Rapperswil 2014.
Straw poll: Encourage continued work?
SF 11, WF 9, N 3, WA 0, SA 0.
Carruth enumerated the following questions, which were encouraged to be answered: should it be ill-formed if this expression has side-effects? Is it an evaluated expression? That changes whether it odr-uses things, whether we have to emit code? Can it be allowed to instantiate templates, and is so what is the point of instantiation?
Discussed in Urbana. EWG voted to forward the proposal to CWG for wording review.
Section: 13.5.6 [over.ref] Status: Open Submitter: Pascal Costanza Opened: 2013-08-23 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3723.html
Discussed in Chicago 2013: Return object that overloads -> to point to subobject. Wrapper that points into itself. Boost "iterator facade" that uses curiously recursive template pattern that does this. It's a pain. What other motivating use cases demonstrate this is a general problem? Has to be a relation to operator. that we've tried to invent a few times. Extra integer looks odd, more so than hack for operator++. We're not yet convinced problem is general enough to require a language change. Not opposed to change, but feels like it's tail end of bigger problem. Didn't quite like the syntax.
Section: 7.6.1 [dcl.attr.grammar] Status: Open Submitter: Walter Brown Opened: 2013-08-30 Last modified: 2015-05-22
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Discussion:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3744.pdf
Discussed in Chicago 2013:
Poll 4: Conservative approach. SF 3 - 2 - 5 - 0 - 0 SA
Poll 5: Aggressive approach: SF 6 - 1 - 3 - 0 - 0 SA
Poll 6: Support both, with different attributes. SF 4 - 2 - 3 - 1 - 0 SA
Vandevoorde expressed that he likes the idea of having two attributes, one for memoizing and the other for non-side-effecting. Stroustrup and Spertus expressed concern about multiple attributes being confusing. Stroustrup gave his guidance to have a two-part proposal with both the "aggressive" mode and the more conservative mode. Vandevoorde promised his assistance for writing the wording.
Section: 7.1.6.4 [dcl.spec.auto] Status: Open Submitter: Peter Gottschling Opened: 2013-08-30 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4035.pdf
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3748.pdf
Reviewed in Chicago 2013. Stroustrup recommended looking into Vandevoorde's solution of doing a type manipulation instead of running code in a conversion since there doesn't seem to a palatable reason for running code, and thought that the solution should be simplified, and it should be much better than what you can hack today. The author is encouraged to pursue the idea further.
Discussed in Rapperswil 2014.
Straw poll: On the proposal as is:
SF 0 - WF 1 - N 8 - WA 13 - SA 2
Stroustrup said his reading of this is that we agree there's a problem, but we don't think that this is the solution.
Straw poll: See a new proposal based on this one, but with "using auto" only applying to variable and data member initializers:
SF 2 - WF 8 - N 8 - WA 1 - SA 3
Carruth suggested writing a new paper with more rationale/explanation of the tradeoffs, decisions, etc., and Dennett agreed.
Section: 3.9 [basic.types] Status: New Submitter: Lawrence Crowl, Bjarne Stroustrup Opened: 2013-10-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3810.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3820.html
See also issue 125.
Section: 1 [intro] Status: New Submitter: Ville Voutilainen Opened: 2014-01-16 Last modified: 2015-05-22
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Discussion:
92 Should exception-specifications be part of the type system? -> (SUPERSEDED) Instruct Core to close 92 as NAD. Maurer points out that he's going to be dealing with the area with TM. The rationale for closing is that EWG doesn't agree that being able to overload on noexcept is worth the trouble, nor does EWG think being able to have pointers-to-functions being different parameter types if they have different noexcept-specifications. -> New paper and a new EWG issue, 169.
476 Determining the buffer size for placement new -> new EWG issue, 90. The EWG issue is NAD -> clarification from CWG requested.
687 template keyword with unqualified-ids -> new EWG issue, 92. -> instruct CWG to reopen, Vandevoorde working on wording.
728 Restrictions on local classes -> new EWG issue, 93.
794 Base-derived conversion in member type of pointer-to-member conversion -> Open an EWG issue for 794, 94. Snyder reports he is interested in authoring a paper.
914 Value-initialization of array types -> Open an EWG issue for 914 and 1300, 96. Instruct Core to close either (probably 914)as a DUP. Instruct Core not to think EWG has looked at 1300, because EWG hasn't. Include 1326 in the EWG issue, leave that open in Core as well.
944 reinterpret_cast for all types with the same size and alignment
1008 Querying the alignment of an object -> new EWG issue, 98.
1048 auto deduction and lambda return type deduction. -> Instruct Core to reopen 1048 and clarify the issue, since there is implementation divergence. Ask Merrill whether he has implemented what he thought was right. clang seems to be consistent. decltype(f()) is const A in gcc, decltype(b()) is A in gcc. in clang, both are const A.
1077 Explicit specializations in non-containing namespaces -> EWG has an issue for 1077. Link to the core issue from that (done), and vice versa if CWG chair so chooses.
1300 T() for array types -> Open an EWG issue for 914 and 1300, 96. Instruct Core to close either (probably 914)as a DUP. Instruct Core not to think EWG has looked at 1300, because EWG hasn't. Include 1326 in the EWG issue, leave that open in Core as well.
1326 Deducing an array bound from an initializer-list -> see above
1331 const mismatch with defaulted copy constructor -> Open an EWG issue for 1331, 101. Needs analysis and implementation vendor feedback.
1393 Pack expansions in using-declarations -> Open an EWG issue for 1393, 102. There are other related extension almost-proposals that should be considered in addition to the case in point.
1426 Allowing additional parameter types in defaulted functions -> new EWG issue, 103.
1433 trailing-return-type and point of declaration -> new EWG issue, 104.
1451 Objects with no linkage in non-type template arguments -> new EWG issue, 105.
1463 extern "C" alias templates -> new EWG issue, 106, see above about 13.
1469 Omitted bound in array new-expression
1555 Language linkage and function type compatibility -> Instruct SG12 to handle 1555. (done, SG12 chair has been notified)
1561 Aggregates with empty base classes -> new EWG issue, 108.
1577 Unnecessary restrictions on partial specializations -> new EWG issue, 110. -> issue is NAD, suggest CWG closes as NAD as well.
1582 Template default arguments and deduction failure -> new EWG issue, 111. -> EWG wishes to send this back to CWG for drafting Spicer's suggestion.
1643 Default arguments for template parameter packs -> Link 1643 to EWG issue 15 (done) and vice versa.
1826 const floating-point in constant expressions
Straw poll, Rapperswil 2014:
To make it consistent by allowing "const float f = 2.0" to allow use of f in constexpr evaluation:
SF 4 - WF 12 - N 1 - WA 0 - SA 0
-> instruct CWG to reopen and resolve so that const floating point is allowed in constant expressions.
1790 Ellipsis following function parameter pack
1864 List-initialization of array objects
1871 Non-identifier characters in ud-suffix
1876 Preventing explicit specialization
1912 exception-specification of defaulted function
1914 Duplicate standard attributes
1915 Potentially-invoked destructors in non-throwing constructors
1931 Default-constructible and copy-assignable closure types
1934 Relaxing exception-specification compatibility requirements
1957 decltype(auto) with direct-list-initialization
2097 Lambdas and noreturn attribute
Section: 6.5 [stmt.iter] Status: Open Submitter: Stephan T. Lavavej Opened: 2014-01-17 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3994.htm
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3853.htm
Issaquah 2014: EWG discussed the paper, and found consensus to move the proposal forward to Core, and it has wording already.
Rejected by the full committee in Urbana. Author is going to revise.
Wording available:
The paper contains the proposed wording.Section: 9.3 [class.mfct] Status: New Submitter: Matthew Fioravante Opened: 2013-12-08 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3863.html
Section: 9.2 [class.mem] Status: Open Submitter: J. D. Garcia, X. Li Opened: 2014-01-16 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4294.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3875.pdf
Issaquah 2014: EWG discussed the paper, and the author was present and received feedback to do further work on the paper.
Section: 1 [intro] Status: Open Submitter: Michael Price Opened: 2014-01-16 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3880.html
Issaquah 2014: The paper was presented to EWG, and the author was encouraged to continue working on the idea and to come up with concrete proposals for EWG to vote on.
Section: 13.5 [over.oper] Status: Open Submitter: Gabriel Dos Reis Opened: 2014-02-14 Last modified: 2015-05-22
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Discussion:
In c++std-core-14770, Dos Reis suggests that operator[] and operator() should both be allowed to be static. In addition to that, he suggests that both should allow multiple parameters. It's well known that there's a possibility that this breaks existing code (foo[1,2] is valid, the thing in brackets is a comma-expression) but there are possibilities to fix such cases (by requiring parens if a comma-expression is desired). EWG should discuss whether such unification is to be strived for.
Discussed in Rapperswil 2014. EWG points out that there are more issues to consider here, in terms of other operators, motivations, connections with captureless lambdas, who knows what else, so an analysis paper is requested.
Section: 5.1.1 [expr.prim.general] Status: Open Submitter: Mihai Rusu Opened: 2008-02-27 Last modified: 2015-05-22
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Discussion:
See Core issue 687.
Discussed in Rapperswil 2014. EWG thinks the use of a template keyword in such cases should be allowed. Vandevoorde to write the wording for CWG review.
Section: 14 [temp] Status: New Submitter: Faisal Vali Opened: 2008-10-05 Last modified: 2015-05-22
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Discussion:
See Core issue 728.
Section: 4.11 [conv.mem] Status: New Submitter: CH, Detlef Vollman Opened: 2009-03-03 Last modified: 2015-05-22
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Discussion:
See Core issue 794. Jeff Snyder reports he is interested in authoring a paper.
Section: 5.2.3 [expr.type.conv] Status: Open Submitter: Gabriel Dos Reis Opened: 2009-06-10 Last modified: 2015-05-22
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Discussion:
See Core issue 914, Core issue 1300 and Core issue 1326.
Discussed in Rapperswil 2014. Dos Reis and Krügler encouraged to write a paper.
Section: 5.3.6 [expr.alignof] Status: Open Submitter: Steve Clamage Opened: 2009-11-27 Last modified: 2015-05-22
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Discussion:
See Core issue 1008.
Discussed in Rapperswil 2014. Vandevoorde to write a paper.
Section: 12.8 [class.copy] Status: Open Submitter: Daniel Krügler Opened: 2011-03-18 Last modified: 2015-05-22
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Discussion:
See Core issue 1331.
Needs analysis and implementation vendor feedback.
Discussed in Rapperswil 2014. Krügler encouraged to write a paper, including motivating examples.
Section: 14.5.3 [temp.variadic] Status: Open Submitter: Daniel Krügler Opened: 2011-09-10 Last modified: 2015-05-22
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Discussion:
See Core issue 1393.
There are other related extension almost-proposals that should be considered in addition to the case in point.
Vandevoorde to write a paper and bring it back to EWG. The guidance is to allow multiple names in using-declarations, and also allowing a pack.
Section: 14 [temp] Status: Open Submitter: Daveed Vandevoorde Opened: 2011-08-19 Last modified: 2015-05-22
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Discussion:
See Core issue 1463 and Core issue 13.
Discussed in Rapperswil 2014. Vandevoorde to write a paper.
Section: 8.5.1 [dcl.init.aggr] Status: Open Submitter: Gabriel Dos Reis, Oleg Smolsky Opened: 2012-09-29 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4404.html
See Core issue 1561.
Discussed in Rapperswil 2014. Daryle Walker has an unpublished paper aiming to solve this here. EWG suggests that Dos Reis works with Walker to get the paper on EWG's plate.
N4404 was discussed in Lenexa, EWG rejected the paper as such and recommended looking at a more comprehensive approach.
Section: 14.8.2 [temp.deduct] Status: Ready Submitter: John Spicer Opened: 2012-10-31 Last modified: 2015-05-22
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Discussion:
See Core issue 1582.
Discussed in Rapperswil 2014. EWG wishes CWG to handle this, and to draft to implement Spicer's suggestion.
Section: 3.8 [basic.life] Status: New Submitter: Lawrence Crowl Opened: 2014-01-20 Last modified: 2015-05-22
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Discussion:
See http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3899.html.
Section: 8.4.2 [dcl.fct.def.default] Status: Open Submitter: Oleg Smolsky, Bjarne Stroustrup, Andrew Tomazos, Mike Spertus Opened: 2014-02-19 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4476.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4475.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4239.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4126.htm
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4175.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4176.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4114.htm
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3950.html
Discussed in Rapperswil 2014.
Straw polls:
Proposal + the ability to default as global
SF: 10 F: 9 N: 2 A: 1 SA: 0
Proposal + the ability to default as global + the generation of negated operators
SF: 5 F: 9 N: 3 A: 1 SA: 2
Terse syntaxes only
SF: 7 F: 2 N: 4 A: 8 SA: 1
Terse syntaxes in addition
SF: 6 F: 6 N: 5 A: 4 SA: 1
Library solution
SF: 0 F: 0 N: 1 A: 12 SA: 7
Mutable members are not special (participate in the comparisons)
SF: 2 F: 6 N: 2 A: 5 SA: 6
Mutable members make defaulting ill-formed
SF: 3 F: 7 N: 6 A: 3 SA: 1
Mutable members do not participate in generated comparisons
SF: 7 F: 3 N: 0 A: 7 SA: 4
Proposal + the ability to default as global + mutables ill-formed
SF: 12 F: 7 N: 0 A: 1 SA: 2
The guidance of the vote is not very clear, but what seems clear is that EWG more or less encouraged the paper author to come up with a revised proposal.
Discussed in Urbana. EWG voiced preference towards an opt-in approach.
Discussed in Lenexa. The design changes that alleviate compatibility concerns gained EWG support. There is ongoing work to produce Core wording, but that work raised further design questions. The design paper needs an update for Kona.
Wording available:
The paper contains the proposed wording.Section: 4 [conv] Status: Open Submitter: Richard Smith Opened: 2014-02-15 Last modified: 2015-05-22
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Discussion:
With core issue 393, we're allowing function parameters to have type 'T (*)[]' or 'T (&)[]'. This has always been allowed for variables, and is useful in practice:
// guaranteed to be passed an actual array, not just a pointer, but the array can be of any size.
void f(int n, int (&a)[]) {
... do stuff with first n elements of a ...
}
extern int arr[];
void g() { f(100, arr); }
void h(int *p) { f(100, p); } // error, not an array!
// elsewhere
int arr[100]; // ok, no problem
void i() { f(100, arr); } // error!
Note that if you actually know the bound of 'arr', you *cannot* pass it to f, because you can't pass 'int [100]' glvalues to a 'int (&)[]' parameter.
So, I'd like EWG to consider this extension:
We should allow "array of N T" glvalues to convert to "array of unknown bound of T", and likewise we should allow "cv pointer to array of N T" prvalues to convert to "cv pointer to array of unknown bound of T" prvalues.
Discussed in Rapperswil 2014. EWG wishes to encourage Smith to write a paper for EWG review, EWG expects to send it forwards to CWG without much ado.
Section: 12.2 [class.temporary] Status: Open Submitter: Geoffrey Romer Opened: 2014-05-31 Last modified: 2015-05-22
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Discussion:
The discussion thread started at [c++std-core-25298]
The core issues are CWG 900 and CWG 1498
Romer wrote:
The current resolutions of CWG 900 and 1498 don't seem right to me. The NAD rationale for CWG 900 appears to be saying that there is no problem, because the lifetime of _expression_ is extended by being bound to a reference. However, that applies only to the value of _expression_ itself; the lifetimes of any proper sub-expressions of _expression_ are not so extended. CWG 900 may be a little ambiguous about which temporaries it's referring to, but CWG 1498 is not; it's quite explicitly concerned with sub-expression temporaries, so the rationale for CWG 900 definitely does not apply.
To make this concrete, consider the following example, taken from https://svn.boost.org/trac/boost/ticket/7630:
std::vector<int> vec;
for (int val : vec | boost::adaptors::reversed
| boost::adaptors::uniqued) {
// Do stuff with val
}
The behavior of this code is undefined, because the temporary returned by the subexpression (vec | boost::adaptors::reversed) is destroyed before the loop body is entered, leaving __range's referent holding a dangling reference to it. The adaptors can mostly fix the issue by overloading on rvalue references, but at the cost of a substantial increase in code complexity.
Smith suggested handling this as an EWG issue.
Discussed in Rapperswil 2014. EWG wants a solution, and welcomes a paper tackling the issue. Vandevoorde raised concerns introducing any new lifetime models. Stroustrup pointed out that the end-of-full-expression rule came about to reduce memory footprint compared to the end-of-block rule, and is good for RAII uses. Is it possible to solve the issue by just modifying the specification of a range-for loop?
Section: 9.6 [class.bit] Status: Open Submitter: S. Davalle, D. Gutson, A. Bustamante Opened: 2014-05-06 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3986.pdf
Wording available:
The paper contains the proposed wording.
Discussed in Rapperswil 2014. Not polled yet.
Garcia thought attributes should be explored as a solution rather than using a bitfield of negative width. Stroustrup asked why the facility is per-field rather than per-class. Gutson explained that he doesn't always want pack whole structures, and Stroustrup and Tomazos pointed out that splitting a larger class into smaller ones should solve the problem.
Vandevoorde pointed out implementation issues and asked what happens if a target architecture doesn't support the resulting padding (or the lack of it). It was suggested that the current standard may allow implementations to diagnose such cases anyway.
Naumann made the point that he thinks code should be portable but that object layout cannot be specified in such a strict manner due to architecture differences. Smolsky asked whether there are plans to impose endianness requirements on the representation of built-in types, and voiced concern about a performance impact.
Wong expressed doubt whether a standard solution is necessary, since every vendor has a solution. Vandevoorde replied that that sounds like a good reason to provide a standard solution.
Bos asked why there needs to be this support in classes rather than manipulating bits with a function. Dennett pointed out that this proposal turns things that aren't bitfields into bitfield-like things, because what looks like an int cannot be treated as an int.
Vandevoorde pointed out that alignas is almost a solution, but it doesn't apply to bitfields, and suggested using arrays instead of plain ints for the data that needs to be packed. Stroustrup recommended doing a survey of existing practice.
Section: 3.9.2 [basic.compound] Status: Open Submitter: J. Snyder, R. Smith Opened: 2014-05-23 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4025.pdf
Discussed in Rapperswil 2014, with a fairly long discussion that didn't yield very concrete conclusions.
Straw polls:
Array of runtime bounds:
SF: 2 F: 2 N: 6 A: 11 SA: 2
Objects of dynamic size:
SF: 1 F: 7 N: 7 A: 6 SA: 2
Magic allocation outside of a type:
SF: 0 F: 11 N: 9 A: 1 SA: 0
Nothing:
SF: 2 F: 10 N: 6 A: 4 SA: 0
The group found it difficult to interpret the results, and where to go from here.
Carruth pointed out that it doesn't seem we can agree on an evolution path to take, and perhaps that should be our message to the full committee, and if someone disagrees, they should come up with a better evolution path. Voutilainen pointed out that this is not the first time we have failed to find real consensus about dynamic arrays. Smith said having nothing is the status quo of the C++ standard, and ARBs are what the TS proposes, and there would be a possibility to keep ARBs in a TS without integrating them to the main standard.
Section: 3.5 [basic.link] Status: Open Submitter: Herb Sutter Opened: 2014-05-23 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4028.pdf
Discussed in Rapperswil 2014
Straw poll: should we encourage Sutter to go ahead:
SF 10 F 8 N 3 A 3 SA 3
Sutter asked the reason for the votes against, Gregor expressed doubt on whether the approach is feasible with regards to the amount of work, Carruth expressed doubt on having two slightly different standard libraries.
Stra poll: language ABI:
SF 12 F 14 N 0 A 1 SA 0
Wong said something like this would be ideal for a Study Group. Wong and Sutter expressed interest in continuing to work on the idea.
Section: 23.3 [sequences] Status: New Submitter: Lawrence Crowl Opened: 2014-05-22 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4043.html
Section: 15 [except] Status: Open Submitter: D. Gutson, A. Bustamante, P. Oliva, M. Diaz Opened: 2014-05-27 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4234.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4049.pdf
Discussed in Urbana. The author is encouraged to work on it further.
Section: 23.3 [sequences] Status: New Submitter: Lawrence Crowl Opened: 2014-05-26 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4050.html
Section: 5.1.2 [expr.prim.lambda] Status: New Submitter: Herb Sutter Opened: 2014-07-02 Last modified: 2015-05-22
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Discussion:
It has been reported that various people have noticed that it's possible to write
auto lambda = []{};
but not
auto lambda2 = [] mutable {};
In the mutable case, parentheses are required, thus:
auto lambda3 = []() mutable {};
The proposed consistency fix is to change the grammar to allow omitting the parentheses for mutable lambdas as well.
An additional question EWG needs to decide is whether the empty parentheses could be omitted for other cases besides mutable, such as
[] -> float {return 42;};
[] noexcept {foo();};
Wording available:
Change the grammar in 5.1.2 [expr.prim.lambda] paragraph 1
lambda-expression:
lambda-introducer lambda-declaratoropt compound-statement
...
lambda-declarator:
( parameter-declaration-clause ) mutableopt
exception-specificationopt attribute-specifier-seqopt trailing-return-typeopt
mutableopt exception-specificationopt attribute-specifier-seqopt trailing-return-typeopt
Remove from 5.1.2 [expr.prim.lambda] paragraph 4
Modify in 5.1.2 [expr.prim.lambda] paragraph 5If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were ().
This function call operator or operator template is declaredconst(9.3.1) if and only if thelambda-expression's parameter-declaration-clause is not followed bylambda-declarator does not contain the keywordmutable.
Section: 14.3 [temp.arg] Status: Open Submitter: Maurice Bos Opened: 2014-05-26 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4072.html
Discussed in Rapperswil 2014.
Vote: encourage the author to come back with a more complete proposal for ...[N]: SF 8 - WF 9 - N 5 - WA 1 - SA 0.
Section: 18.1 [support.general] Status: Ready Submitter: Andrew Tomazos Opened: 2014-07-04 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4121.pdf
Discussed in Urbana. The design was approved by EWG, and the author was encouraged to proceed to LEWG and CWG.
Section: X [contract] Status: Open Submitter: J. Lakos, A. Zakharov, A. Beels, N. Myers Opened: 2014-10-09 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4415.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4435.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4378.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4379.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4253.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4135.pdf
Discussed at length in Urbana. In general, various paper authors are encourage to do further work together on contract programming. See also issue 137, 159 and 168.
Discussed in Lenexa. EWG's guidance was to allow contracts on both declarations and definitions, and to unify the proposals.
Section: 7.1.2 [dcl.fct.spec] Status: Ready Submitter: Hal Finkel, Richard Smith, David Krauss Opened: 2014-09-15 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4424.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4147.pdf
Discussed in Urbana. EWG encouraged solving the problem, and wanted the shared constants to be separated from expression aliases, so that the former can be solved sooner than the latter.
N4424 was discussed in Lenexa and accepted by EWG. The authors are encouraged to take a wording paper to CWG.
Section: 3.9 [basic.types] Status: Open Submitter: Matheus Izvekov Opened: 2014-09-24 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4148.html
Discussed in Urbana. The author is encouraged to revise the paper. EWG suggested splitting the core language concept of trivially copyable into trivially copyable and trivially movable, and gave guidance against adding new library traits for the latter, since such a trait would not have use cases.
Section: 8.3.5 [dcl.fct] Status: New Submitter: Andrzej Krzemienski Opened: 2014-10-03 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4160.html
Section: 5.2.2 [expr.call] Status: Open Submitter: Herb Sutter, Bjarne Stroustrup Opened: 2014-10-04 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4474.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4165.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4174.pdf
Discussed in Urbana. EWG favored being able to invoke non-members with the member call syntax, and was undecided on the other aspects. Authors are encouraged to follow up.
Discussed in Lenexa. EWG didn't reach consensus on the proposal. Straw poll numbers were SF/WF/N/WA/SA 9/8/3/4/6. There is strong opposition to the proposal with concerns about maintainability of code when new free functions are added.
Section: 18.9 [support.initlist] Status: Open Submitter: David Krauss Opened: 2014-10-06 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4166.pdf
Discussed in Urbana. EWG thought the problem is worth solving, but expressed doubt about this proposal being the right approach. Further work is encouraged.
Section: 13.5 [over.oper] Status: Open Submitter: B. Stroustrup, G. Dos Reis Opened: 2014-10-11 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4477.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4173.pdf
Discussed in Urbana, the authors are encouraged to continue working further on it.
Discussed in Lenexa. The author is encouraged to revise and come back to EWG with a paper that has wording in it, the overall design was accepted by EWG.
Section: 8.4.3 [dcl.fct.def.delete] Status: Open Submitter: Matthias Kretz Opened: 2014-10-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4186.pdf
Discussed in Urbana. EWG thought the problem is worth solving, and the author is encouraged to follow up.
Section: 9.2 [class.mem] Status: Open Submitter: L. Deniau, A. Naumann Opened: 2014-10-01 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4188.pdf
Discussed in Urbana. The proposed approach was found to have some issues, authors are encouraged to continue to try and find a solution for runtime-sized types.
Section: 5 [expr] Status: Open Submitter: G. Dos Reis, H. Sutter, J. Caves Opened: 2014-10-13 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4228.pdf
Discussed in Urbana. Author is encouraged to pursue further, with the aim of adopting this proposal into C++17.
Section: 3.8 [basic.life] Status: Open Submitter: Pablo Halpern Opened: 2014-10-12 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4393.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4158.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4034.pdf
Discussed in Urbana. The author is encouraged to revise.
Discussed again in Lenexa. Great concerns about the impact on lifetime, progress postponed until the paper author and some experts review whether there are solutions that do not require language changes.
Section: 3.6.1 [basic.start.main] Status: Open Submitter: D. Diaz, E. Bringas, D. Gutson, J. Maurer Opened: 2014-10-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4357.html
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4226.pdf
Discussed in Urbana. EWG was supportive of the idea, CWG pointed out various issues related to multiple translation units. Authors are encouraged to revise.
Section: X [modules] Status: Open Submitter: G. Dos Reis, M. Hall, G. Nishanov Opened: 2014-05-27 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4466.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4465.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4047.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4214.pdf
Discussed in Urbana. Authors are encouraged to work further, and implementation vendors are encouraged to gain implementation experience with the proposed syntax and semantics.
Discussed in Lenexa. The overall notational model was supported by EWG, but there are fairly many technical details to solve. The authors are encouraged to talk to other implementation vendors before Kona and update the design paper.
Section: 18.1 [support.general] Status: Ready Submitter: Robert Douglas Opened: 2014-10-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4129.pdf
Discussed in Urbana. EWG voted to forward the proposal to LEWG and CWG.
Section: 20.8.5 [comparisons] Status: Ready Submitter: Gabriel Dos Reis Opened: 2014-10-13 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4229.pdf
Discussed in Urbana. EWG voted to forward the proposal to LWG.
Section: X [coroutine] Status: New Submitter: Gor Nishanov, Jim Radigan Opened: 2014-11-11 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4286.pdf
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4287.pdf
Discussed in Urbana. Further proposals expected.
Discussed in Lenexa, EWG gave guidance related to what sort of keywords to use, and recommended continuing the work on the wording. The design is accepted, but the library-magic parts have been requested to be taken through LEWG.
Section: 4 [conv] Status: Open Submitter: Jens Maurer Opened: 2014-11-20 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2014/n4320.html
Discussed in Lenexa, accepted by EWG. The motion was postponed to Kona in the full committee, the library and ABI impact needs to be studied further.
Section: 2.12 [lex.key] Status: Ready Submitter: Alisdair Meredith Opened: 2014-11-26 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4340.html
Discussed in Lenexa. EWG supported making the use of register invalid but keeping it as a reserved word. The author is to go to Core with wording.
Section: 5.1.3 [expr.prim.fold] Status: Open Submitter: Thibaut Le Jehan Opened: 2015-01-20 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4358.pdf
Discussed in Lenexa. EWG was interested in fixing some parts of the problem, but didn't agree to the paper as presented. The author is encouraged to talk to the authors of the fold expression design (Sutton, Smith) and revise.
Section: 8.3.5 [dcl.fct] Status: Open Submitter: Douglas Boffey Opened: 2015-01-22 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4360.pdf
Discussed in Lenexa. Summary of straw polls:
Straw poll: Are you intrigued by a notion of delayed parameter evaluation?
SF: 6 F: 7 N: 2 A: 3 SA: 1
Straw poll: Annotation at the call site?
SF: 1 F: 0 N: 9 A: 0 SA: 7
Straw poll: Pass by lambda rather than evaluate on first use?
SF: 5 F: 4 N: 5 A: 0 SA: 3
EWG expressed interest in the idea, but various people were very concerned about the impact of such a facility, voicing caution that it is not just syntactic sugar but rather something much more significant, and suggested that a much more comprehensive analysis paper should be produced.
Section: 5.9 [expr.rel] Status: Open Submitter: Lawrence Crowl Opened: 2015-02-08 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4367.html
Discussed in Lenexa. EWG didn't think this approach should block the generated operators proposal (see 116), and thought that the library parts look interesting. The author is encouraged to continue progress on the library parts.
Section: 14.1 [temp.param] Status: New Submitter: RS Opened: 2014-10-11 Last modified: 2015-05-22
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Discussion:
According to 14.1 [temp.param] paragraph 4,
A non-type template-parameter shall have one of the following (optionally cv-qualified) types: - integral or enumeration type, - pointer to object or pointer to function, - lvalue reference to object or lvalue reference to function, - pointer to member, - std::nullptr_t.
Since void* isn't a pointer-to-object type, this prohibits using them as template non-type parameters, which disallows constructs like
template<typename T, std::enable_if_t<std::is_array<T>::value>* =
nullptr>
void foo(T&) {}
(Though the above does compile in Clang, GCC, and MSVC with no warnings.)
There doesn't seem to be a good reason for this prohibition. While a void* template parameter can only have a single value (the null pointer), the same is true for a non-type parameter of type std::nullptr_t, which is allowed.
Wording available:
Change the second bullet point of 14.1 [temp.param] paragraph 4 as indicated:
-pointer to object or pointer to functionobject pointer or function pointer,
Change the second bullet point of 14.3.2 [temp.arg.nontype] paragraph 5 as indicated:
- for a non-type template-parameter oftype pointer to objectobject pointer type, qualification conversions...
Section: 18.9 [support.initlist] Status: New Submitter: David Krauss Opened: 2014-09-30 Last modified: 2015-05-22
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Discussion:
See LWG 2432.
Section: 7.3.3 [namespace.udecl] Status: Ready Submitter: Richard Smith Opened: 2015-04-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4429.html
Discussed in Lenexa. The paper was accepted, the author is to take the wording to CWG.
Section: 7 [dcl.dcl] Status: Open Submitter: Michael Price Opened: 2015-04-09 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4433.html
Discussed in Lenexa. EWG expressed interest in the paper, but thought that it might benefit from other facilities that are currently considered, including reflection facilities. The author is encouraged to continue working on the problem.
Section: X [concepts] Status: Open Submitter: Walter Brown Opened: 2015-04-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4434.pdf
FI 14:
It seems unfortunate that concepts cannot be declared as members of class templates. This seemingly makes it impossible to define concepts for constraining multiple template parameter packs (if concepts as static member functions were possible, one could provide e.g. two packs so that the class template gets the first pack and the member function template gets the second. That can't be done with a namespace-scope concept because multiple packs in function templates require deduction, and concepts don't take arguments that could be deduced.). As an example, practical needs for such constraining arise in standard library implementations, when constraining the variadic converting constructors of std::tuple.
US 13:
The following requirement seems overly restrictive, as it can be fairly easily (but tediously) be worked around:
A requires-expression shall appear only within a concept definition (7.1.7), or within the requires-clause of a template-declaration (Clause 14) or function declaration (8.3.5).(The tedious workaround for each concept C is to define an overload set consisting of two function templates, one unconstrained and returning false, the other constrained by C and returning true.
US 15/16:
We believe that (generally speaking) the non-throwing of exceptions is a part of the runtime contract of a function, not something that should be advertised in the type system outside a few very specific cases related to move operations. As a 'requires' expression is always free to invoke the 'noexcept' operator to produce a predicate, we believe that is sufficient support for exception constraints in the language, and directly supporting this additional term in the grammar would be harmful, encouraging compile-time contracts taking away an important library implementer freedom. As the TS is intended to provide feedback, we believe it would be better to proceed without this, and see how much demand arises from using the alternate form, and whether that alternate form alone is too cumbersome for real world use.
If we retain exception constraints, the optional noexcept specifier should support the full range of the noexcept grammar.
US 24:
The syntactic distinction between a function concept and a variable concept seems to serve no useful purpose. A single concept syntax seems sufficient, and especially so once redundant elements are removed.
US 36:
This paragraph introduces implicit conversion constraints to specify (via the trailing-return-type notation -> ) that a constraint is satisfied iff an expression E is convertible to a type T. It would be very useful to have similar constraints that are satisfied iff decltype(E) is exactly the type T.
Discussed in Lenexa. There was interest in going forward with the design of Concepts as-is, and open EWG issues for various comments for future discussion. This issue tracks those comments/issues.
Section: 20.6.9.1 [allocator.members] Status: Ready Submitter: Ville Voutilainen Opened: 2015-04-07 Last modified: 2015-05-22
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Discussion:
See LWG 2089.
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4462.html
Discussed in Lenexa. EWG didn't see any need for a language change, and considered the library approach to be ok. Jonathan Wakely is writing an LWG proposal to resolve the library issue.
Section: 15.4 [except.spec] Status: Open Submitter: Ville Voutilainen Opened: 2015-04-10 Last modified: 2015-05-22
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Discussion:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4473.html
Discussed in Lenexa. EWG's guidance was to modify the original wording by Merrill so that it would talk about potentially-evaluated full-expressions rather than unevaluated contexts. The author is encouraged to revise and come back.
Section: 4 [conv] Status: New Submitter: Zhihao Yuan Opened: 2015-04-30 Last modified: 2015-05-22
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Discussion:
See LEWG bug 95.
Or consider a language feature for that?
auto(y) // cast to itself
Section: 9 [class] Status: New Submitter: Michael Reilly Opened: 2015-05-21 Last modified: 2015-05-22
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Discussion:
Proposes to allow unions with non-static data members of differing access control to be considered a standard-layout union, provided they meet the other criteria of such.
The project that motivated this defect report is located here:
http://preshing.com/20150324/safe-bitfields-in-cpp/
https://github.com/preshing/cpp11-on-multicore/blob/master/common/bitfield.h
Originally, the union had both public and private members. This caused the union to be not standard-layout, which in turn caused the program to technically have undefined behavior.
I argue that this defect is important to address because there seems to be only one guarantee about the behavior of unions when accessing the "wrong" member, and this guarantee only applies to standard-layout unions. So if the definition of a standard-layout union is too strict then it can cause undefined behavior in places where it strictly does not need to be.