2018-07-06

Richard Smith (editor) (Google Inc)

Thomas Köppe (co-editor) (Google DeepMind)

Jens Maurer (co-editor)

Dawn Perchik (co-editor) (Bright Side Computing, LLC)

Special thanks to Casey Carter for supplying a pull request to merge

[P0898R3](http://wg21.link/p0898r3) (LWG motion 28),

and working with us on editorial cleanups within the

12 pages of added text.

Thanks to all those who have [submitted editorial

issues](https://github.com/cplusplus/draft/wiki/How-to-submit-an-editorial-issue)

and to those who have provided pull requests with fixes.

* [N4762](http://wg21.link/n4762) is the current working draft for C++20. It replaces [N4750](http://wg21.link/n4750).

* N4764 is this Editors' Report.

CWG motion 1: [Core issue resolutions](http://wg21.link/p1113r0) for 6 issues in "ready" status applied:

* [2254](http://wg21.link/cwg2254) Standard-layout classes and bit-fields

* [2293](http://wg21.link/cwg2293) Requirements for *simple-template-id* used as a *class-name*

* [2294](http://wg21.link/cwg2294) Dependent `auto` static data members

* [2321](http://wg21.link/cwg2321) Conditional operator and cv-qualified class prvalues

* [2322](http://wg21.link/cwg2322) Substitution failure and lexical order

* [2339](http://wg21.link/cwg2339) Underspecified template arguments in structured bindings

CWG motion 2: [Core issue resolutions](http://wg21.link/p1114r0) for 6 issues in "tentatively ready" status applied:

* [2233](http://wg21.link/cwg2233) Function parameter packs following default arguments

* [2249](http://wg21.link/cwg2249) *identifier*s and *id-expression*s

* [2285](http://wg21.link/cwg2285) Issues with structured bindings

* [2351](http://wg21.link/cwg2351) `void{}`

* [2356](http://wg21.link/cwg2356) Base class copy and move constructors should not be inherited

* [2359](http://wg21.link/cwg2359) Unintended copy initialization with designated initializers **(not a DR)**

CWG motion 3: [P0806R2 "Deprecate implicit capture of `this` via `[=]`"](http://wg21.link/p0806r2)

CWG motion 4: [P1042R1 `__VA_OPT__` wording clarifications"](http://wg21.link/p1042r1)

CWG motion 5: [P0929R2 "Checking for abstract class types"](http://wg21.link/p0929r2) applied, resolving 2 core issues:

* [1640](http://wg21.link/cwg1640) Array of abstract instance of class template

* [1646](http://wg21.link/cwg1646) *decltype-specifier*s, abstract classes, and deduction failure

CWG motion 6: [P0732R2 "Class types in non-type template parameters"](http://wg21.link/p0732r2)

CWG motion 7 was not approved

CWG motion 8: [P1025R1 "Update the reference to the Unicode standard"](http://wg21.link/p1025r1)

CWG motion 9: [P0528R3 "The curious case of padding bits, featuring atomic compare-and-exchange"](http://wg21.link/p0528r3)

CWG motion 10: [P0722R3 "Efficient sized delete for variable sized classes"](http://wg21.link/p0722r3)

CWG motion 11: [P1064R0 "Allowing virtual function calls in constant expressions"](http://wg21.link/p1064r0)

CWG motion 12: [P1008R1 "Prohibit aggregates with user-declared constructors"](http://wg21.link/p1008r1)

CWG motion 13: [P1120R0 "Consistency improvements for `<=>` and other comparison operators"](http://wg21.link/p1120r0)

CWG motion 14: [P0542R5 "Contract-based programming"](http://wg21.link/p0542r5) **see below**

CWG motion 15: [P0941R2 "Feature-test macros"](http://wg21.link/p0941r2) **see below**

CWG motion 16: [P0892R2 "`explicit(bool)`"](http://wg21.link/p0892r2)

LWG motions 1-5 apply to the Parallelism TS

LWG motions 6 and 7 apply to the Reflection TS

LWG motions 8 and 9 apply to the Coroutines TS

LWG motion 10 applies to the Networking TS

LWG motion 11: [Library issue resolutions](http://wg21.link/p1082r0) for 14 issues in "Ready" and "Tentatively Ready" status applied:

* [2139](http://wg21.link/lwg2139) What is a user-defined type? **see below**

* [2970](http://wg21.link/lwg2970) Return type of `std::visit` misspecified

* [3058](http://wg21.link/lwg3058) Parallel `adjacent_difference` shouldn't require creating temporaries

* [3062](http://wg21.link/lwg3062) Unnecessary `decay_t` in `is_execution_policy_v` should be `remove_cvref_t`

* [3067](http://wg21.link/lwg3067) `recursive_directory_iterator::pop` must invalidate

* [3074](http://wg21.link/lwg3074) Non-member functions for `valarray` should only deduce from the `valarray`

* [3076](http://wg21.link/lwg3076) `basic_string` CTAD ambiguity

* [3079](http://wg21.link/lwg3079) [LWG 2935](http://wg21.link/lwg2935) forgot to fix the `existing_p` overloads of `create_directory`

* [3080](http://wg21.link/lwg3080) Floating point `from_chars` pattern specification breaks round-tripping

* [3083](http://wg21.link/lwg3083) What should `ios::iword(-1)` do?

* [3094](http://wg21.link/lwg3094) [time.duration.io]p4 makes surprising claims about encoding

* [3100](http://wg21.link/lwg3100) Unnecessary and confusing "empty span" wording

* [3102](http://wg21.link/lwg3102) Clarify `span` `iterator` and `const_iterator` behavior

* [3104](http://wg21.link/lwg3104) Fixing `duration` division

* Resolution of [3071](http://wg21.link/lwg3071) (`read_until` still refers to

"input sequence") from P1082R0 was not part of this motion, as it applies to

the Networking TS.

LWG motion 12: [Library issue resolution](http://wg21.link/p0475r1) for 1 issue applied:

* [2511](http://wg21.link/lwg2511) Guaranteed copy elision for piecewise construction

LWG motion 13: [P0476R2 "Bit-casting object representations"](http://wg21.link/p0476r2)

LWG motion 14: [P0788R3 "Standard library specification in a concepts and contracts world"](http://wg21.link/p0788r3)

LWG motion 15 was not approved

LWG motion 16: [P0458R2 "Checking for existence of an element in associative containers"](http://wg21.link/p0458r2)

LWG motion 17: [P0759R1 "`fpos` requirements"](http://wg21.link/p0759r1)

LWG motion 18: [P1023R0 "`constexpr` comparison operators for `std::array`"](http://wg21.link/p1023r0)

LWG motion 19: [P0769R2 "Add `shift` to `<algorithm>`"](http://wg21.link/p0769r2)

LWG motion 20: [P0887R1 "The `identity` metafunction"](http://wg21.link/p0887r1)

LWG motion 21: [P0879R0 "`constexpr` for `swap` and `swap`-related functions"](http://wg21.link/p0879r0) applied, resolving 1 issue:

* [2800](http://wg21.link/lwg2800) `constexpr` `swap`

LWG motion 22: [P0758R1 "Implicit conversion traits and utility functions"](http://wg21.link/p0758r1)

LWG motion 23: [P0556R3 "Integral power-of-2 operations"](http://wg21.link/p0556r3)

LWG motion 24: [P0019R8 "`atomic_ref`"](http://wg21.link/p0019r8)

LWG motion 25: [P0935R0 "Eradicating unnecessarily explicit default constructors from the standard library"](http://wg21.link/p0935r0)

LWG motion 26: [P0646R1 "Improving the return value of `erase`-like algorithms"](http://wg21.link/p0646r1)

LWG motion 27: [P0619R4 "Reviewing deprecated facilities of C++17 for C++20"](http://wg21.link/p0619r4) **see below**

LWG motion 28: [P0898R3 "Standard library concepts"](http://wg21.link/p0898r3) **see below**

Subclause structure and paragraph order of [dcl.attr.contracts] was reworked.

Several normatively-redundant statements were converted to notes or removed.

Multiple papers moved at this meeting included suggested feature-test macros.

However, these were not presented as editing instructions, because we did not

have feature-test macros in the standard wording yet. After consultation with

CWG and LWG, the following feature test macros have been added in addition to

those listed in CWG motion 15 (all with value `201806L`):

* `__has_cpp_attribute(assert)`

* `__has_cpp_attribute(ensures)`

* `__has_cpp_attribute(expects)`

* `__cpp_explicit_bool`

* `__cpp_nontype_template_parameter_class`

* `__cpp_lib_atomic_ref`

* `__cpp_lib_bit_cast`

* `__cpp_lib_concepts`

* `__cpp_lib_constexpr_swap_algorithms`

* `__cpp_lib_list_remove_return_type`

CWG motion 16 would have us change wording that was deleted by CWG motion 12.

The requested change in CWG motion 16 (from teletype "`explicit`" to body font "explicit")

was ignored.

The underlying text has changed between the drafting of the resolution to this

issue and its application. We believe every requested edit has been applied;

however, an additional use of "user-defined type" has been added to

[namespace.std] that should likely also have been covered by this wording.

It has *not* been changed.

These motions both introduce a `<bit>` header, but the organizational structure

suggested by motion 23 doesn't make sense for the functionality

added by motion 13.

The wording of motion 23 has been rearranged to fit into

the structure established by motion 13.

Added the new `contains` member function for associative containers to the list

of member function templates that do not participate in overload resolution

unless the comparator is transparent, after consultation with LWG.

LWG motion 19 adds a `shift_right` algorithm to `<algorithm>`

as a non-`constexpr` function template.

LWG motion 21 instructs that we mark

all non-parallel algorithms in `<algorithm>`

as `constexpr`.

Naturally, however,

its proposed wording change does not list the `shift_right` algorithm.

After consultation with LWG, `shift_right` has been marked `constexpr`

to satisfy the intent of LWG motion 21.

Synopses for

`<ccomplex>`,

`<cstdalign>`,

`<cstdbool>`, and

were not removed;

instead, they have been repurposed as synopses for

`<complex.h>`,

`<stdalign.h>`,

`<stdbool.h>`, and

`<tgmath.h>`.

(The latter used to be defined in terms of the former,

but can no longer be specified in that way.)

Also introduced a synopsis for `<iso646.h>`.

Clause labels shortened and simplified throughout.

The single-item clauses [concept.movable], [concept.copyable],

[concept.semiregular], and [concept.regular] have been merged into their

parent, [concepts.object].

The single-item clauses [concept.signed.int] and [concept.unsigned.int]

have been merged into their parent, [concept.integral].

Legacy concept names changed from Cpp98Something to Cpp17Something. Many of

these concepts did not exist in C++98 (which in any case was revoked and

replaced by C++03).

Reworked "uniform random bit generator" requirements to describe them in terms

of the UniformRandomBitGenerator concept.

Removed "there need be no subsumption relationship" wording that is already

implied by the core language rules for concepts.

The claue reorganization described in P1076R1 and discussed at the WG21

Rapperswil meeting has been applied to the working draft, with the following

modifications:

[class.copy] was left containing only two paragraphs of text, neither of which

had any normative impact. It has been removed.

The top-level subclauses of [algorithms] have been reordered to make the

description of the `<numeric>` header fit better into its structure.

Several paragraphs of [class] describing various properties of classes have

been moved into a new subclause [class.prop] "Properties of clases".

The single-item subclauses

[move.iter.op=] and

[move.iter.op.const]

have been merged into [move.iter.cons].

The single-item subclauses

[move.iter.op.star],

[move.iter.op.ref], and

[move.iter.op.index]

have been merged into [move.iter.elem].

The single-item subclauses

[move.iter.op.+],

[move.iter.op.-],

[move.iter.op.incr],

[move.iter.op.+=],

[move.iter.op.decr], and

[move.iter.op.-=]

have been merged into [move.iter.nav].

The now-empty [move.iter.ops] has been removed.

A log of editorial fixes made to the working draft since N4750 is below.

This list excludes changes

that do not affect the body text or only affect whitespace or typeface. For a

complete list including such changes (or for the actual deltas applied by these

changes), consult the [draft sources on github](https://github.com/cplusplus/draft/compare/n4750...n4762).