P1152R4.bs

<pre class='metadata'>
Title: Deprecating <code>volatile</code>
Shortname: P1152
Revision: 4
Audience: CWG
Status: P
Group: WG21
URL: http://wg21.link/P1152R4
!Source: <a href="https://github.com/jfbastien/papers/blob/master/source/P1152R4.bs">github.com/jfbastien/papers/blob/master/source/P1152R4.bs</a>
Editor: JF Bastien, Apple, jfbastien@apple.com
No abstract: true
Date: 2019-07-19
Markup Shorthands: markdown yes
</pre>

<style>
th { vertical-align: middle; }
</style>

Abstract {#abstract}
========

We propose deprecating most of `volatile`. See [[#word]] for the details.

The proposed deprecation preserves the useful parts of `volatile`, and removes
the dubious / already broken ones. This paper aims at breaking at compile-time
code which is today subtly broken at runtime or through a compiler update. The
paper might also break another type of code: that which doesn't exist. This
removes a significant foot-gun and removes unintuitive corner cases from the
languages.

The first version of this paper, [[P1152R0]], has extensive background
information which is not repeated here:

  * <a href="https://wg21.link/P1152R0#over">Overview</a>
  * <a href="https://wg21.link/P1152R0#where">How we got here</a>
  * <a href="https://wg21.link/P1152R0#why">Why the proposed changes?</a>
  * <a href="https://wg21.link/P1152R0#examples">Examples</a>

See [[P1382R0]] for the follow-up paper on `volatile_load<T>` /
`volatile_store<T>` requested by SG1.

In Cologne, CWG was able to review this paper but LWG was not. The library parts
of this paper have therefore moved to [[P1831R0]] so that the language changes
can make C++20, and the library changes can be added to C++20 later.


Edit History {#edit}
============

r3 → r4 {#r3r4}
-------

Edit wording of the following sections as suggested by Jens Maurer:
[**expr.post.incr**], [**expr.pre.incr**], [**expr.ass**], [**dcl.fct**],
[**over.load**], [**over.built**].

Drop one word from [**tuple**] as suggested by Arthur O'Dwyer.

Add [**dcl.struct.bind**].

Add Annex D.

Move library parts to [[P1831R0]].


r2 → r3 {#r2r3}
-------

[[P1152R2]] was reviewed offline by Alisdair Meredith.

  * `is_volatile<T>` should have been `is_volatile_v<T>`.
  * Use *Mandates* instead of *Constraints* to allow marking a function as deleted or employing a `static_assert` instead of using `enable_if` or `requires`. This likely provides better error messages.
  * "`is_volatile_v<T>` is `false`" was erroneously used instead of testing whether the `this` pointer itself was `volatile`. Wording of the *Mandates* clause was updated to only apply to the `volatile` overloads of methods instead.


r1 → r2 {#r1r2}
-------

[[P1152R1]] was seen by SG1 and EWG in Kona. This update does the following:

  * Also edit sections [**expr.post.incr**], [**expr.pre.incr**], [**expr.ass**], which are redundant with other sections already modified by this paper.
  * Change *Remarks* to *Constraints* per [[P1369R0]].
  * Change language wording to explicitly call out deprecation.

<table>
    <thead>
        <tr>
            <th>Poll</th>
            <th>Group</th>
            <th>SF</th>
            <th>F</th>
            <th>N</th>
            <th>A</th>
            <th>SA</th>
            <th>Outcome</th>
        </tr>
    </thead>
    <tr>
        <th>Forward this paper—with edits as discussed—to EWG for C++20.</th>
        <th>SG1</th><th>3</th><th>12</th><th>1</th><th>1</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Proposal as presented for C++20.</th>
        <th>EWG</th><th>6</th><th>27</th><th>1</th><th>0</th><th>0</th><th>✅</th>
    </tr>
</table>

r0 → r1 {#r0r1}
-------

[[P1152R0]] was seen by SG1 and EWG in San Diego. This update does the
following:

  * Remove background information from the paper.
  * Follow the guidance from SG1 and EWG, based on the polls below.

<table>
    <thead>
        <tr>
            <th>Poll</th>
            <th>Group</th>
            <th>SF</th>
            <th>F</th>
            <th>N</th>
            <th>A</th>
            <th>SA</th>
            <th>Outcome</th>
        </tr>
    </thead>
    <tr>
        <th>Deprecate `volatile` compound operations (including `++` and `--`) on scalar types (arithmetic, pointer, enumeration).</th>
        <th>SG1</th><th>4</th><th>19</th><th>3</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Deprecate `volatile` compound operations (including `++` and `--`) on scalar types (arithmetic, pointer, enumeration).</th>
        <th>EWG</th><th>4</th><th>9</th><th>4</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Deprecate usage of `volatile` assignment chaining on scalar types (arithmetic, pointer, enumeration, pointer to members, `nullptr_t`).</th>
        <th>SG1</th><th>6</th><th>15</th><th>3</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Deprecate usage of `volatile` assignment chaining on scalar types (arithmetic, pointer, enumeration, pointer to members, `nullptr_t`).</th>
        <th>EWG</th><th>6</th><th>9</th><th>3</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>SG1 would be OK if we deprecated `volatile`-qualified member functions (pending separate decision on what we do with `volatile` atomic).</th>
        <th>SG1</th><th>1</th><th>5</th><th>10</th><th>4</th><th>3</th><th>❌</th>
    </tr>
    <tr>
        <th>EWG would be OK if we deprecated `volatile`-qualified member functions (pending separate decision on what we do with `volatile` atomic).</th>
        <th>EWG</th><th>2</th><th>7</th><th>7</th><th>1</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>SG1 would be OK if we deprecated `volatile` partial template specializations, overloads, or qualified member functions in the STL for all but the atomic, `numeric_limits`, and type traits (`remove_volatile`, `add_volatile`, etc) parts of the Library.</th>
        <th>SG1</th><th>1</th><th>9</th><th>6</th><th>2</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>EWG would be OK if we deprecated `volatile` partial template specializations, overloads, or qualified member functions in the STL for all but the atomic, `numeric_limits`, and type traits (`remove_volatile`, `add_volatile`, etc) parts of the Library.</th>
        <th>EWG</th><th>1</th><th>11</th><th>9</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Deprecate `volatile` member functions of atomic in favor of new template partial specializations which will only declare load, store, and only exist when `is_always_lock_free` is true.</th>
        <th>SG1</th><th>2</th><th>1</th><th>1</th><th>11</th><th>2</th><th>❌</th>
    </tr>
    <tr>
        <th>Deprecate `volatile` member functions of atomic in favor of new template partial specializations which will only declare load, store, RMW, and only exist when `is_always_lock_free` is true.</th>
        <th>SG1</th><th>4</th><th>7</th><th>3</th><th>3</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Deprecate `volatile` member functions of atomic in favor of new template partial specializations which will only declare load, store, RMW, and only exist when `is_always_lock_free` is true.</th>
        <th>EWG</th><th>2</th><th>9</th><th>3</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>Deprecate `volatile` member functions of atomic in favor of new template partial specializations which will only declare load, store, RMW.</th>
        <th>SG1</th><th>0</th><th>0</th><th>0</th><th>10</th><th>7</th><th>❌</th>
    </tr>
    <tr>
        <th>SG1 would be OK if we deprecated top-level `volatile` parameters.</th>
        <th>SG1</th><th>6</th><th>9</th><th>6</th><th>2</th><th>1</th><th>✅</th>
    </tr>
    <tr>
        <th>EWG would be OK if we deprecated top-level `volatile` parameters.</th>
        <th>EWG</th><th>6</th><th>9</th><th>6</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>EWG would be OK if we deprecated top-level const parameters.</th>
        <th>EWG</th><th>0</th><th>2</th><th>5</th><th>8</th><th>8</th><th>❌</th>
    </tr>
    <tr>
        <th>SG1 would be OK if we deprecated top-level `volatile` return values.</th>
        <th>SG1</th><th>6</th><th>9</th><th>4</th><th>2</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>EWG would be OK if we deprecated top-level `volatile` return values.</th>
        <th>EWG</th><th>6</th><th>6</th><th>5</th><th>0</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>EWG would be OK if we deprecated top-level const return values.</th>
        <th>EWG</th><th>2</th><th>3</th><th>3</th><th>5</th><th>5</th><th>❌</th>
    </tr>
    <tr>
        <th>SG1 interested is interested in hearing about `volatile_load<T>` / `volatile_store<T>` free functions in a separate paper, given that time is limited and we could be doing something else.</th>
        <th>SG1</th><th>0</th><th>17</th><th>4</th><th>3</th><th>0</th><th>✅</th>
    </tr>
    <tr>
        <th>EWG interested is interested in hearing about `volatile_load<T>` / `volatile_store<T>` free functions in a separate paper, given that time is limited and we could be doing something else.</th>
        <th>EWG</th><th>2</th><th>11</th><th>4</th><th>1</th><th>0</th><th>✅</th>
    </tr>
</table>


Wording {#word}
=======

The proposed wording follows the language and library approach to deprecation:


  * Language deprecation is called out in the Standard text itself, and repeated
    in Annex D.
  * Library deprecation presents the library without the deprecated feature, and
    only mentions said feature in Annex D.

Program execution [**intro.execution**] {#intro.execution}
---------------------------------------

No changes.
  
<blockquote>

  Accesses through `volatile` glvalues are evaluated strictly according to the
  rules of the abstract machine.
  
  Reading an object designated by a `volatile` glvalue, modifying an object,
  calling a library I/O function, or calling a function that does any of those
  operations are all *side effects*, which are changes in the state of the
  execution environment. *Evaluation* of an expression (or a subexpression) in
  general includes both value computations (including determining the identity
  of an object for glvalue evaluation and fetching a value previously assigned
  to an object for prvalue evaluation) and initiation of side effects. When a
  call to a library I/O function returns or an access through a `volatile`
  glvalue is evaluated the side effect is considered complete, even though some
  external actions implied by the call (such as the I/O itself) or by the
  `volatile` access may not have completed yet.

</blockquote>

Data races [**intro.races**] {#data.races}
----------------------------
  
No changes.

<blockquote>

  Two accesses to the same object of type `volatile` `std::sig_atomic_t` do not
  result in a data race if both occur in the same thread, even if one or more
  occurs in a signal handler. For each signal handler invocation, evaluations
  performed by the thread invoking a signal handler can be divided into two
  groups A and B, such that no evaluations in B happen before evaluations in A,
  and the evaluations of such `volatile` `std::sig_atomic_t` objects take values
  as though all evaluations in A happened before the execution of the signal
  handler and the execution of the signal handler happened before all
  evaluations in B.
  
</blockquote>

Forward progress [**intro.progress**] {#intro.progress}
-------------------------------------
  
No changes.

<blockquote>

  The implementation may assume that any thread will eventually do one of the
  following:

  * terminate,
  * make a call to a library I/O function,
  * perform an access through a `volatile` glvalue, or
  * perform a synchronization operation or an atomic operation

  During the execution of a thread of execution, each of the following is termed
  an execution step:
  
  * termination of the thread of execution,
  * performing an access through a `volatile` glvalue, or
  * completion of a call to a library I/O function, a synchronization
    operation, or an atomic operation.

</blockquote>

Increment and decrement [**expr.post.incr**] {#expr.post.incr}
--------------------------------------------

Modify as follows.

<blockquote>

  The value of a postfix `++` expression is the value of its operand. [ *Note:*
  The value obtained is a copy of the original value —*end note* ] The operand
  shall be a modifiable lvalue. The type of the operand shall be an
  arithmetic type other than *cv* `bool`, or a
  pointer to a complete object type. <ins>An operand with `volatile`-qualified
  type is deprecated; see [**depr.volatile.type**].</ins> The value of the operand object is
  modified by adding `1` to it. The value computation of the `++` expression is
  sequenced before the modification of the operand object. With respect to an
  indeterminately-sequenced function call, the operation of postfix `++` is a
  single evaluation. [ *Note:* Therefore, a function call shall not intervene
  between the lvalue-to-rvalue conversion and the side effect associated with
  any single postfix `++` operator. —*end note* ] The result is a prvalue. The
  type of the result is the *cv*-unqualified version of the type of the operand.
  If the operand is a bit-field that cannot represent the incremented value, the
  resulting value of the bit-field is implementation-defined. See also
  [**expr.add**] and [**expr.ass**].

  The operand of postfix `--` is decremented analogously to the postfix `++`
  operator. [ *Note:* For prefix increment and decrement, see
  [**expr.pre.incr**]. —*end note* ]

</blockquote>

Class member access [**expr.ref**] {#expr.ref}
----------------------------------

No changes.
  
<blockquote>

  Abbreviating *postfix-expression.id-expression* as `E1.E2`, `E1` is called the
  *object expression*. If `E2` is a bit-field, `E1.E2` is a bit-field. The type
  and value category of `E1.E2` are determined as follows. In the remainder of
  [**expr.ref**], *cq* represents either `const` or the absence of `const` and
  *vq* represents either `volatile` or the absence of `volatile`. *cv*
  represents an arbitrary set of cv-qualifiers.
  
  * If `E2` is a non-static data member and the type of `E1` is “*cq1 vq1 X*”,
    and the type of `E2` is “*cq2 vq2 T*”, the expression designates the named
    member of the object designated by the first expression. If `E1` is an
    lvalue, then `E1.E2` is an lvalue; otherwise `E1.E2` is an xvalue. Let the
    notation *vq12* stand for the “union” of *vq1* and *vq2*; that is, if *vq1*
    or *vq2* is `volatile`, then *vq12* is `volatile`. Similarly, let the
    notation *cq12* stand for the “union” of *cq1* and *cq2*; that is, if *cq1*
    or *cq2* is `const`, then *cq12* is `const`. If `E2` is declared to be a
    `mutable` member, then the type of `E1.E2` is “*vq12 T*”. If `E2` is not
    declared to be a `mutable` member, then the type of `E1.E2` is
    “*cq12 vq12 T*”.

</blockquote>

Increment and decrement [**expr.pre.incr**] {#expr.pre.incr}
-------------------------------------------

Modify as follows.

<blockquote>

  The operand of prefix `++` is modified by adding `1`. The operand shall be a
  modifiable lvalue. The type of the operand shall be an arithmetic type
  other than *cv* `bool`, or a pointer to a
  completely-defined object type. <ins>An operand with `volatile`-qualified type
  is deprecated; see [**depr.volatile.type**].</ins> The result is the updated operand; it is an
  lvalue, and it is a bit-field if the operand is a bit-field. The expression
  `++x` is equivalent to `x+=1`. [ *Note:* See the discussions of [**expr.add**]
  and assignment operators [**expr.ass**] for information on conversions. —*end
  note* ]

  The operand of prefix `--` is modified by subtracting `1`. The requirements on
  the operand of prefix `--` and the properties of its result are otherwise the
  same as those of prefix `++`. [ *Note:* For postfix increment and decrement,
  see [**expr.post.incr**]. —*end note* ]

</blockquote>

Assignment and compound assignment operators [**expr.ass**] {#expr.ass}
-----------------------------------------------------------

Modify as follows.

<blockquote>

  <ol>
  
  <li>
  The assignment operator (`=`) and the compound assignment operators all group
  right-to-left.
  </li>

  <li>
  All require a modifiable lvalue as their left operand; their result is an
  lvalue referring to the left operand. The result in all cases is a bit-field
  if the left operand is a bit-field. In all cases, the assignment is sequenced
  after the value computation of the right and left operands, and before the
  value computation of the assignment expression. The right operand is sequenced
  before the left operand. With respect to an indeterminately-sequenced function
  call, the operation of a compound assignment is a single evaluation. [ *Note:*
  Therefore, a function call shall not intervene between the lvalue-to-rvalue
  conversion and the side effect associated with any single compound assignment
  operator. —*end note* ]
  
  <pre>
  <em>assignment-expression</em>
      <em>conditional-expression</em>
      <em>logical-or-expression assignment-operator initializer-clause</em>
      <em>throw-expression</em>
  
  <em>assignment-operator</em>: one of
      <code>=  *=  /=  %=   +=  -=  >>=  <<=  &=  ^=  |=</code>
  </pre>
  </li>

  <li>
  In simple assignment (`=`), the object referred to by the left operand is
  modified by replacing its value with the result of the right operand.
  </li>

  <li>
  If the left operand is not of class type, the expression is implicitly
  converted to the cv-unqualified type of the left operand.
  </li>
  
  <li>
  If the left operand is of class type, the class shall be complete. Assignment
  to objects of a class is defined by the copy/move assignment.
  </li>

  <li>
  [ *Note:* For class objects, assignment is not in general the same as
  initialization. —*end note* ]
  </li>

  <li>
  When the left operand of an assignment operator is a bit-field that cannot
  represent the value of the expression, the resulting value of the bit-field is
  implementation-defined.
  </li>
  
  <li><ins>
  Simple assignments where the left operand is a `volatile`-qualified type that
  is not of class type are deprecated (see [**depr.volatile.type**]) unless they are
  either a discarded-value expression or appear in an unevaluated context.
  </ins></li>

  <li>
  The behavior of an expression of the form
  <code>E1 <em>op</em>= E2</code> is equivalent to
  <code>E1 = E1 <em>op</em> E2</code>
  except that `E1` is evaluated only once. <ins>Such expressions are deprecated
  if `E1` has `volatile`-qualified type;
  see [**depr.volatile.type**].</ins> In `+=` and `-=`, `E1` shall either
  have arithmetic type or be a pointer to a possibly cv-qualified
  completely-defined object type. In all other cases, `E1` shall have arithmetic
  type.
  </li>

  <li>
  If the value being stored in an object is read via another object that
  overlaps in any way the storage of the first object, then the overlap shall be
  exact and the two objects shall have the same type, otherwise the behavior is
  undefined. [ *Note:* This restriction applies to the relationship between the
  left and right sides of the assignment operation; it is not a statement about
  how the target of the assignment may be aliased in general.
  See [**basic.lval**]. —*end note* ]
  </li>

  <li>
  A *braced-init-list* may appear on the right-hand side of

    <ol>
      <li>an assignment to a scalar, in which case the initializer list shall have at most a single element. The meaning of `x = {v}`, where `T` is the scalar type of the expression `x`, is that of `x = T{v}`. The meaning of `x = {}` is `x = T{}`.
      <li>an assignment to an object of class type, in which case the initializer list is passed as the argument to the assignment operator function selected by overload resolution.
    </ol>
  </li>
  
  </ol>

</blockquote>

The cv-qualifiers [**dcl.type.cv**] {#dcl.type.cv}
-----------------------------------

No changes.
  
<blockquote>

  The semantics of an access through a `volatile` glvalue are
  implementation-defined. If an attempt is made to access an object defined with
  a `volatile`-qualified type through the use of a non-`volatile` glvalue, the
  behavior is undefined.

  [ *Note:* `volatile` is a hint to the implementation to avoid aggressive
  optimization involving the object because the value of the object might be
  changed by means undetectable by an implementation. Furthermore, for some
  implementations, `volatile` might indicate that special hardware instructions
  are required to access the object. See [**intro.execution**] for detailed
  semantics. In general, the semantics of `volatile` are intended to be the same
  in C++ as they are in C. —*end note* ]

</blockquote>

Functions [**dcl.fct**] {#dcl.fct}
-----------------------

Modify as follows.

<blockquote>

  The *parameter-declaration-clause* determines the arguments that can be
  specified, and their processing, when the function is called. [ *Note:* The
  *parameter-declaration-clause* is used to convert the arguments specified on
  the function call; see [**expr.call**] —*end note* ] If the
  *parameter-declaration-clause* is empty, the function takes no arguments. A
  parameter list consisting of a single unnamed parameter of non-dependent type
  `void` is equivalent to an empty parameter list. Except for this special case,
  a parameter shall not have type *cv* `void`. <ins>A parameter with
  `volatile`-qualified type is deprecated; see [**depr.volatile.type**].</ins>
  If the *parameter-declaration-clause* terminates with an ellipsis or a
  function parameter pack, the number of arguments shall be equal to or greater
  than the number of parameters that do not have a default argument and are not
  function parameter packs. Where syntactically correct and where "`...`" is not
  part of an *abstract-declarator*, "`, ...`" is synonymous with "`...`".
  
  [...]
  
  The type of a function is determined using the following rules. The type of
  each parameter (including function parameter packs) is determined from its own
  *decl-specifier-seq* and *declarator*. After determining the type of each
  parameter, any parameter of type "array of `T`" or of function type `T` is
  adjusted to be "pointer to `T`". After producing the list of parameter types,
  any top-level *cv-qualifier*s modifying a parameter type are deleted when
  forming the function type. The resulting list of transformed parameter types
  and the presence or absence of the ellipsis or a function parameter pack is
  the function's *parameter-type-list*.
  
  [...]
  
  Functions shall not have a return type of type array or function, although
  they may have a return type of type pointer or reference to such things. There
  shall be no arrays of functions, although there can be arrays of pointers to
  functions.
  
  <ins>A `volatile`-qualified return type is
  deprecated; see [**depr.volatile.type**].</ins>

</blockquote>

Structured binding declarations [**dcl.struct.bind**] {#dcl.struct.bind}
-----------------------------------------------------

Modify as follows:

<blockquote>

A structured binding declaration introduces the *identifier*s
<code>v<sub>0</sub></code>, <code>v<sub>1</sub></code>,
<code>v<sub>2</sub></code>, ... of the *identifier-list* as names of *structured
binding*s. Let *cv* denote the *cv-qualifier*s in the *decl-specifier-seq* and
*S* consist of the *storage-class-specifier*s of the *decl-specifier-seq* (if
any). <ins>A *cv* that includes `volatile` is deprecated; see
[**depr.volatile.type**].</ins> First, a variable with a unique name `e` is
introduced. If the *assignment-expression* in the *initializer* has array type
`A` and no *ref-qualifier* is present, `e` is defined by

*attribute-specifier-seq*<sub>*opt*</sub> *S* *cv* `A` `e` `;`

</blockquote>

Non-static member functions [**class.mfct.non-static**] {#class.mfct.non-static}
-------------------------------------------------------

No changes.

<blockquote>

  A non-static member function may be declared `const`, `volatile`, or
  `const volatile`. These cv-qualifiers affect the type of the `this` pointer.
  They also affect the function type of the member function; a member
  function declared `const` is a `const` member function, a member function declared
  `volatile` is a `volatile` member function and a member function declared
  `const volatile` is a `const volatile` member function.

</blockquote>

The this pointer [**class.this**] {#class.this}
---------------------------------

No changes.

<blockquote>

   In the body of a non-static member function, the keyword `this` is a prvalue
   expression whose value is the address of the object for which the function is
   called. The type of `this` in a member function of a class `X` is `X*`. If
   the member function is declared `const`, the type of `this` is `const X*`, if
   the member function is declared `volatile`, the type of `this` is
   `volatile X*`, and if the member function is declared `const volatile`, the type of
   `this` is `const volatile X*`.
   
   `volatile` semantics apply in `volatile` member functions when accessing the
   object and its non-static data members.

</blockquote>

Constructors [**class.ctor**] {#class.ctor}
-----------------------------

No changes.

<blockquote>

  A constructor can be invoked for a `const`, `volatile` or `const volatile`
  object. `const` and `volatile` semantics are not applied on an object under
  construction. They come into effect when the constructor for the most derived
  object ends.

</blockquote>

Destructors [**class.dtor**] {#class.dtor}
----------------------------

No changes.

<blockquote>

  A destructor is used to destroy objects of its class type. The address of a
  destructor shall not be taken. A destructor can be invoked for a `const`,
  `volatile` or `const volatile` object. `const` and `volatile` semantics are
  not applied on an object under destruction. They stop being in effect when the
  destructor for the most derived object starts.

</blockquote>

Overloadable declarations [**over.load**] {#over.load}
-----------------------------------------

No changes since [**dcl.fct**] already handles deprecation.

<blockquote>

  Parameter declarations that differ only in the presence or absence of
  `const` and/or `volatile` are equivalent. That is, the
  `const` and `volatile` type-specifiers for each parameter type are
  ignored when determining which function is being declared, defined, or called.

</blockquote>

Built-in operators [**over.built**] {#over.built}
-----------------------------------

No changes since [**expr.post.incr**], [**expr.pre.incr**], and [**expr.ass**]
already handle deprecation.

<blockquote>

  In the remainder of this section, *vq* represents either `volatile` or no
  cv-qualifier.
  
  For every pair (*T*, *vq*), where *T* is an arithmetic type other than `bool`,
  there exist candidate operator functions of the form

  <pre><code>
  vq T &amp; operator++(vq T &amp;);
  T operator++(vq T &amp;, int);
  </code></pre>

  For every pair (*T*, *vq*), where *T* is an arithmetic type other than `bool`,
  there exist candidate operator functions of the form

  <pre><code>
  vq T &amp; operator--(vq T &amp;);
  T operator--(vq T &amp;, int);
  </code></pre>

  For every pair (*T*, *vq*), where *T* is a cv-qualified or cv-unqualified
  object type, there exist candidate operator functions of the form

  <pre><code>
  T*vq&amp; operator++(T*vq&amp;);
  T*vq&amp; operator--(T*vq&amp;);
  T* operator++(T*vq&amp;, int);
  T* operator--(T*vq&amp;, int);
  </code></pre>
  
  For every quintuple (*C1*, *C2*, *T*, *cv1*, *cv2*), where *C2* is a class
  type, *C1* is the same type as *C2* or is a derived class of *C2*, and *T* is an
  object type or a function type, there exist candidate operator functions of
  the form

  <pre><code>
  <em>cv12</em> T&amp; operator->*(<em>cv1</em> C1*, <em>cv2</em> T C2::*);
  </code></pre>
  
  For every triple (*L*, *vq*, *R*), where *L* is an arithmetic type, and *R* is
  a promoted arithmetic type, there exist candidate operator functions of the
  form

  <pre><code>
  vq L&amp; operator=(vq L&amp;, R);
  vq L&amp; operator*=(vq L&amp;, R);
  vq L&amp; operator/=(vq L&amp;, R);
  vq L&amp; operator+=(vq L&amp;, R);
  vq L&amp; operator-=(vq L&amp;, R);
  </code></pre>
    
  For every pair (*T*, *vq*), where *T* is any type, there exist candidate
  operator functions of the form
  
  <pre><code>
  T*vq&amp; operator=(T*vq&amp;, T*);
  </code></pre>

  For every pair (*T*, *vq*), where *T* is an enumeration or pointer to member
  type, there exist candidate operator functions of the form

  <pre><code>
  vq T&amp; operator=(vq T&amp;, T );
  </code></pre>

  For every pair (*T*, *vq*), where *T* is a cv-qualified or cv-unqualified
  object type, there exist candidate operator functions of the form

  <pre><code>
  T*vq&amp; operator+=(T*vq&amp;, std::ptrdiff_t);
  T*vq&amp; operator-=(T*vq&amp;, std::ptrdiff_t);
  </code></pre>

  For every triple (*L*, *vq*, *R*), where *L* is an integral type, and *R* is a
  promoted integral type, there exist candidate operator functions of the form

  <pre><code>
  vq L&amp; operator%=(vq L&amp;, R);
  vq L&amp; operator<<=(vq L&amp;, R);
  vq L&amp; operator>>=(vq L&amp;, R);
  vq L&amp; operator&amp;=(vq L&amp;, R);
  vq L&amp; operator^=(vq L&amp;, R);
  vq L&amp; operator|=(vq L&amp;, R);
  </code></pre>

</blockquote>


Annex D {#annexD}
-------

Add the following wording to Annex D:

### Deprecated `volatile` types [**depr.volatile.type**] ### {#depr.volatile.type}

Postfix `++` and `--` expressions ([**expr.post.incr**]) and prefix `++` and
`--` expressions ([**expr.pre.incr**]) of `volatile`-qualified arithmetic and
pointer types are deprecated.

Certain assignments where the left operand is a `volatile`-qualified non-class
type are deprecated; see [**expr.ass**].

A function type ([**dcl.fct**]) with a parameter with `volatile`-qualified type
or with a `volatile`-qualified return type is deprecated.

A structured binding ([**dcl.struct.bind**]) of a `volatile`-qualified type is
deprecated.


<pre class=biblio>
{
    "P1831R0": {
        "href": "https://wg21.link/P1831R0",
        "title": "Deprecating volatile: library",
        "authors": ["JF Bastien"],
        "date": "19 July 2019"
    }
}
</pre>