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Attributes for namespace aliases, template parameters, and lambda captures
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<table id="header">
  <tr>
    <th>Document Number:</th>
    <td>P3324R0</td>
  </tr>
  <tr>
    <th>Date:</th>
    <td>2024-10-14</td>
  </tr>
  <tr>
    <th>Audience:</th>
    <td>EWG</td>
  </tr>
  <tr>
    <th>Reply-to:</th>
    <td>Tom Honermann &lt;tom@honermann.net&gt;</td>
  </tr>
</table>


<h1>Attributes for namespace aliases, template parameters, and lambda captures</h1>


<ul>
  <li><a href="#introduction">Introduction</a></li>
  <li><a href="#motivation">Motivation</a></li>
  <li><a href="#design">Design considerations</a>
    <ul>
      <li><a href="#design-namespace">Namespace alias definitions</a></li>
      <li><a href="#design-templates">Template parameter declarations</a></li>
      <li><a href="#design-captures">Lambda captures</a></li>
    </ul>
  </li>
  <li><a href="#proposal">Proposal</a></li>
  <li><a href="#implementation-exp">Implementation experience</a></li>
  <li><a href="#ack">Acknowledgements</a></li>
  <li><a href="#references">References</a></li>
  <li><a href="#wording">Wording</a></li>
</ul>


<h1 id="introduction">Introduction</h1>

<p>
As evidenced by
<a href="http://eel.is/c++draft/gram">annex A (Grammar summary)</a>
in the C++ standard, an attribute may appertain to almost every kind of
declared name or entity including:
<ul>
  <li>Type aliases</li>
  <li>Enumerations</li>
  <li>Enumerators</li>
  <li>Variables (including the unnamed object for structured bindings)</li>
  <li>Structured bindings</li>
  <li>Classes</li>
  <li>Data members</li>
  <li>Functions and function types</li>
  <li>Parameters (function parameters, non-type template parameters,
      exception handlers)</li>
  <li>Concepts</li>
  <li>Labels</li>
  <li>Namespaces</li>
  <li>Modules</li>
</ul>
Exceptions include:
<ul>
  <li>Namespace aliases</li>
  <li>Type template parameters</li>
  <li>Template template parameters</li>
  <li>Lambda captures</li>
</ul>
It is not clear why attributes are not supported for the items in the
exceptions list.
There are existing standard attributes that appear to be relevant to these
items based on their specification or stated recommended practice.
</p>

<p>
The recommended practice for the <tt>[[deprecated]]</tt> attribute in
<a href="http://eel.is/c++draft/dcl.attr.deprecated#4">[dcl.attr.deprecated]p4</a>
seems applicable to namespace aliases:
<blockquote class="quote">
<em>Recommended practice:</em> Implementations should use the
<tt>deprecated</tt> attribute to produce a diagnostic message in case the
program refers to a name or entity other than to declare it, after a declaration
that specifies the attribute.
The diagnostic message should include the text provided within the
<a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-argument-clause"><i>attribute-argument-clause</i></a>
of any <tt>deprecated</tt> attribute applied to the name or entity.
The value of a
<a href="http://eel.is/c++draft/cpp.cond#nt:has-attribute-expression"><i>has-attribute-expression</i></a>
for the <tt>deprecated</tt> attribute should be <tt>0</tt> unless the
implementation can issue such diagnostic messages.
</p>

<p>
</blockquote>
The recommended practice for the <tt>[[maybe_unused]]</tt> attribute in
<a href="http://eel.is/c++draft/dcl.attr.unused#4">[dcl.attr.unused]p4</a>
seems applicable to template parameters and lambda captures.
<blockquote class="quote">
<em>Recommended practice:</em> For an entity marked <tt>maybe_unused</tt>,
implementations should not emit a warning that the entity or its structured
bindings (if any) are used or unused.
For a structured binding declaration not marked <tt>maybe_unused</tt>,
implementations should not emit such a warning unless all of its structured
bindings are unused.
For a label to which <tt>maybe_unused</tt> is applied, implementations should
not emit a warning that the label is used or unused.
The value of a
<a href="http://eel.is/c++draft/cpp.cond#nt:has-attribute-expression"><i>has-attribute-expression</i></a>
for the <tt>maybe_unused</tt> attribute should be <tt>0</tt> if the attribute
does not cause suppression of such warnings.
</blockquote>
</p>

<p>
</blockquote>
The <tt>[[no_unique_address]]</tt> attribute applies to non-static data members
and therefore seems applicable to unnamed non-static data members implicitly
declared in lambda closure types for captured entities.
From
<a href="https://eel.is/c++draft/dcl.attr.nouniqueaddr#1">[dcl.attr.nouniqueaddr]p1</a>:
<blockquote class="quote">
The
<a href="https://eel.is/c++draft/dcl.attr.grammar#nt:attribute-token"><i>attribute-token</i></a>
<tt>no_unique_address</tt> specifies that a non-static data member is a
potentially-overlapping subobject
(<a href="https://eel.is/c++draft/intro.object">[intro.object]</a>).
No
<a href="https://eel.is/c++draft/dcl.attr.grammar#nt:attribute-argument-clause"><i>attribute-argument-clause</i></a>
shall be present.  The attribute may appertain to a non-static data member
other than a bit-field.
</blockquote>
</p>

<p>
The <tt>[[deprecated]]</tt> attribute was adopted for C++14 via
<a title="[[deprecated]] attribute"
   href="https://wg21.link/n3760">
N3760</a>
<sup><a title="[[deprecated]] attribute"
        href="#ref_n3760">[N3760]</a></sup>.
Neither that paper, its previous revision, nor the minutes from EWG and CWG
review during the following meetings demonstrate consideration for the use of
the attribute with a namespace alias.
<ul>
  <li><a href="https://wiki.edg.com/bin/view/Wg21portland2012/EvolutionWorkingGroup">EWG review of N3394 in Portland, 2012</a></li>
  <li><a href="https://wiki.edg.com/bin/view/Wg21chicago2013/CoreWorkingGroup">CWG review of N3760 in Chicago, 2013</a></li>
</ul>
</p>

<p>
The <tt>[[maybe_unused]]</tt> attribute was adopted for C++17 via
<a title="Wording for [[maybe_unused]] attribute."
   href="https://wg21.link/p0212r1">
P0212R1</a>
<sup><a title="Wording for [[maybe_unused]] attribute."
        href="#ref_p0212r1">[P0212R1]</a></sup>.
Neither that paper, its previous revision, the rationale provided in the
previous
<a title="Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes."
   href="https://wg21.link/p0068r0">
P0068R0</a>
<sup><a title="Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes."
        href="#ref_p0068r0">[P0068R0]</a></sup>,
nor the minutes from EWG and CWG review during the following meetings
demonstrate consideration for use of the attribute with a
type template parameter,
template template parameter, or
lambda capture.
<ul>
  <li><a href="https://wiki.edg.com/bin/view/Wg21kona2015/P0068R0">EWG review of P0068R0 in Kona, 2015</a></li>
  <li><a href="https://wiki.edg.com/bin/view/Wg21jacksonville/CoreWorkingGroup">CWG review of D0212R1 in Jacksonville, 2016</a></li>
</ul>
</p>

<p>
The <tt>[[no_unique_address]]</tt> attribute was adopted for C++20 via
<a title="Language support for empty objects"
   href="https://wg21.link/p0840r2">
P0840R2</a>
<sup><a title="Language support for empty objects"
        href="#ref_p0840r2">[P0840R2]</a></sup>.
Neither that paper, the motivation provided in its original R0 revision, nor
the minutes from EWG and CWG review during the following meetings demonstrate
consideration for the use of the attribute with a lambda closure.
<ul>
  <li><a href="https://wiki.edg.com/bin/view/Wg21albuquerque/P0840R0">EWG review of P0840R0 in Albuquerque, 2017</a></li>
  <li><a href="https://wiki.edg.com/bin/view/Wg21jacksonville2018/CoreWorkingGroup">CWG review of P0840R1 and D0840R2 in Jacksonville, 2018</a></li>
</ul>
</p>

<p>
A search of both active and closed CWG issues failed to identify any existing
issues that concern attribute appertainment for the excluded items.
</p>

<p>
There does not appear to be a recorded rationale for omitting attribute support
for the excluded items.
</p>

<p>
The C++ grammar permits an optional sequence of attributes in the declaration of
a non-type template parameter by way of
<a href="http://eel.is/c++draft/temp.param#nt:template-parameter"><i>template-parameter</i></a>
and
<a href="http://eel.is/c++draft/dcl.fct#nt:parameter-declaration"><i>parameter-declaration</i></a>.
The latter directly permits an attribute sequence at the beginning of a
parameter declaration and an attribute sequence is indirectly permitted
following a parameter name by
<a href="http://eel.is/c++draft/dcl.decl.general#nt:noptr-declarator"<i>noptr-declarator</i></a>.
However, some implementations, Clang and EDG among them, diagnose an attribute
that appertains to a non-type template parameter in at least some cases.
The gcc and Microsoft Visual C++ compilers accept such attributes.
<a href="https://godbolt.org/z/G61qK7oPc">https://godbolt.org/z/G61qK7oPc</a>.
<blockquote class="code">
template&lt;[[maybe_unused]] int nttp&gt; // Rejected by EDG and Clang, accepted by gcc and MSVC.
void ft1();
template&lt;int nttp [[maybe_unused]]&gt; // Rejected by EDG, accepted by Clang, gcc, and MSVC.
void ft2();
</blockquote>
</p>


<h1 id="motivation">Motivation</h1>

<p>
Per
<a href="http://eel.is/c++draft/dcl.attr.deprecated#2">[dcl.attr.deprecated]p2</a>,
the <tt>deprecated</tt> attribute is explicitly specified as being able to
appertain to a namespace.
<blockquote class="quote">
The attribute may be applied to the declaration of a class, a
<a href="http://eel.is/c++draft/dcl.typedef#nt:typedef-name">typedef-name</a>,
a variable, a non-static data member, a function, a namespace, an enumeration,
an enumerator, a concept, or a template specialization.
</blockquote>
The inability to apply the <tt>deprecated</tt> attribute to a namespace alias
seems to be an oversight as there is no principled reason for namespace aliases
to never be deprecated.
At present, Clang, gcc, EDG, and Microsoft Visual C++ all reject an attribute
sequence on a namespace alias either before the identifier at a position
consistent with attributes on namespace declarations, or after the identifier.
<a href="https://godbolt.org/z/8oqah7T15">https://godbolt.org/z/8oqah7T15</a>
<blockquote class="code">
namespace ns {}
namespace [[deprecated]] nsa1 = ns; // Rejected by Clang, EDG, gcc, and MSVC.
namespace nsa2 [[deprecated]] = ns; // Rejected by Clang, EDG, gcc, and MSVC.
</blockquote>
</p>

<p>
Per
<a href="http://eel.is/c++draft/dcl.attr.unused#2">[dcl.attr.unused]p2</a>,
the <tt>maybe_unused</tt> attribute is specified as applicable to non-static
data members.
<blockquote class="quote">
The attribute may be applied to the declaration of a class,
<a href="http://eel.is/c++draft/dcl.typedef#nt:typedef-name">typedef-name</a>,
variable (including a structured binding declaration), structured binding,
non-static data member, function, enumeration, or enumerator, or to an
<a href="http://eel.is/c++draft/lex.name#nt:identifier">identifier</a> label (
<a href="http://eel.is/c++draft/stmt.label">[stmt.label]</a>).
</blockquote>
Likewise for the <tt>no_unique_address</tt> attribute per
<a href="https://eel.is/c++draft/dcl.attr.nouniqueaddr#1">[dcl.attr.nouniqueaddr]p1</a>:
<blockquote class="quote">
The
<a href="https://eel.is/c++draft/dcl.attr.grammar#nt:attribute-token"><i>attribute-token</i></a>
<tt>no_unique_address</tt> specifies that a non-static data member is a
potentially-overlapping subobject
(<a href="https://eel.is/c++draft/intro.object">[intro.object]</a>).
No
<a href="https://eel.is/c++draft/dcl.attr.grammar#nt:attribute-argument-clause"><i>attribute-argument-clause</i></a>
shall be present.  The attribute may appertain to a non-static data member
other than a bit-field.
</blockquote>
A lambda capture is not formally a non-static data member, though references to
a lambda capture are usually translated to an access of a non-static data member
of the closure type.
As such, most attributes that may appertain to a non-static data member are
likely to be applicable to lambda captures.
For example, the <tt>maybe_unused</tt> attribute could be used to mark a
lambda capture as intentionally unused.
Such unused captures can be useful to acquire resources such as locks or to
preserve the layout of a closure type when a previously needed capture is no
longer needed.
Likewise, the <tt>no_unique_address</tt> attribute could be used to request a
more optimal class layout for a lambda closure type.
</p>

<p>
At present, Clang issues a warning under <tt>-Wall</tt> for unused lambda
captures.
<a href="http://eel.is/c++draft/stmt.label">http://eel.is/c++draft/stmt.label</a>.
Clang, gcc, EDG, and Microsoft Visual C++ all reject the following code.
<a href="https://godbolt.org/z/4xq1YKodz">https://godbolt.org/z/4xq1YKodz</a>
<blockquote class="code">
namespace std {
  struct mutex {};
  template&lt;typename... Ts&gt;
  struct scoped_lock {
    scoped_lock(Ts...);
  };
}

void f(int p1, int p2) {
  std::mutex m1, m2;
  [ [[maybe_unused]] p1 ]{}();
  [ p2 [[maybe_unused]] ]{}();
  [ [[maybe_unused]] l1 = std::scoped_lock(m1) ]{}();
  [ l2 [[maybe_unused]] = std::scoped_lock(m2) ]{}();
}
</blockquote>
</p>

<p>
None of Clang, gcc, EDG, or Microsoft Visual C++ currently issue warnings for
unused template parameters and this is unlikely to change given how frequently
template parameters are unused.
However, for consistency and to allow for explicit documentation, it is
reasonable to allow the <tt>maybe_unused</tt> attribute to appertain to all
kinds of template parameters.
<a href="https://godbolt.org/z/YbE9cTPvh">https://godbolt.org/z/YbE9cTPvh</a>
<blockquote class="code">
template&lt;[[maybe_unused]] int nttp&gt; // Rejected by EDG and Clang, accepted by gcc and MSVC.
void ft1();
template&lt;int nttp [[maybe_unused]]&gt; // Rejected by EDG, accepted by Clang, gcc, and MSVC.
void ft2();
template&lt;[[maybe_unused]] typename ttp&gt; // Rejected by Clang, EDG, gcc, and MSVC.
void ft3();
template&lt;typename ttp [[maybe_unused]]&gt; // Rejected by Clang, EDG, gcc, and MSVC.
void ft4();
template&lt;[[maybe_unused]] template&lt;typename&gt; class tttp&gt; // Rejected by Clang, EDG, gcc, and MSVC.
void ft5();
template&lt;template&lt;typename&gt; class tttp [[maybe_unused]]&gt; // Rejected by Clang, EDG, gcc, and MSVC.
void ft6();
</blockquote>
</p>

<p>
Absent compelling rationale otherwise, precedent indicates that new kinds of
declared names or entities should allow for attribute appertainment and their
respective declarations should therefore include syntactic locations for an
attribute specifier sequence.
</p>


<h1 id="design">Design considerations</h1>

<p>
There are various and conflicting precedents for where an attribute specifier
sequence is syntactically placed in existing declarations.
<ol>
  <li>In general, an attribute specifier sequence immediately follows the
      identifier in a declaration for an attributed entity.
      Examples include declarations of type aliases, enumerators, variables,
      structured bindings, data members, functions, parameters, concepts, and
      modules.
      Exceptions include label, class, and enum declarations and
      namespace definitions.</li>
  <li><a href="http://eel.is/c++draft/namespace.def.general#nt:namespace-definition"><i>namespace-definition</i></a>
      does not permit an attribute specifier sequence following an identifier
      (inconsistent with the general case) but does permit one following the
      <tt>namespace</tt> keyword (presumably to allow an attribute to be
      specified for an unnamed namespace definition).</li>
  <li><a href="http://eel.is/c++draft/dcl.fct#nt:parameter-declaration"><i>parameter-declaration</i></a>
      permits an attribute specifier sequence following a declarator ID
      (consistent with the general case) and at the start of a parameter
      declaration (presumably to allow an attribute to be specified for an
      unnamed parameter declaration).</li>
</ol>
</p>


<h2 id="design-namespace">Namespace alias definitions</h2>

<p>
Namespace alias definitions always include an identifier.
This presents a choice of:
<ol>
  <li>following the precedent for namespace definitions and allowing an
      attribute specifier sequence between the <tt>namespace</tt> keyword and
      the identifier, or</li>
  <li>following the general precedent and allowing an attribute specifier
      sequence following the identifier, or</li>
  <li>both.</li>
</ol>
No motivation for supporting both has been identified.
Option 1 is proposed based on the rationale that consistency with namespace
definitions is more important than consistency with a general rule that already
has existing exceptions.
</p>


<h2 id="design-templates">Template parameter declarations</h2>

<p>
Type template parameters and template template parameters may be unnamed.
This presents a choice of:
<ol>
  <li>following the precedent for non-type template parameters and function
      parameters and allowing an attribute specifier sequence both at the
      beginning of a declaration and following an identifier, or</li>
  <li>allowing an attribute specifier sequence only at the beginning of a
      declaration, or</li>
  <li>allowing an attribute specifier sequence following an identifier with no
      provision for specifying attributes for unnamed parameters.</li>
</ol>
Option 1 is proposed based on the rationale that consistency with non-type
template parameters and function parameters is more important than avoiding
the cognitive and implementation complexity of supporting multiple locations.
</p>


<h2 id="design-captures">Lambda captures</h2>

<p>
A lambda capture serves two purposes; it grants access to a local variable
from within a lambda expression and it notionally declares a non-static
data member of the closure type.
An attribute applied to a lambda capture can therefore be considered to
appertain to either the capture itself or to the notional non-static data
member.
</p>

<p>
Consider the two lambda expressions in the following example.
The first, <tt>l1</tt>, explicitly captures <tt>i</tt> but does not use it.
The second, <tt>l2</tt>, has a default capture, but no variables are implicitly
captured.
Depending on one's intuition, the attribute might be intended to suppress a
diagnostic about an unused capture as opposed to an unused capture declaration.
<blockquote class="code">
void f(int i) {
  // Don't warn that 'i' is captured but not used?
  auto l1 [i [[maybe_unused]] ] {
    return 0;
  };
  // Don't warn that the default capture declaration was not used?
  auto l2 [= [[maybe_unused]] ] {
    return 0;
  };
}
</blockquote>
This proposal specifies that the attribute appertains to the notional
non-static data member on the basis that doing so is more useful.
For example, <tt>[[no_unique_address]]</tt> has a clear and unambiguous meaning
for the notional non-static data member, but would otherwise be meaningless
for a capture declaration.
As explained below, this proposal locates the new attribute specifier sequence
following each capture declaration.
Should motivation arise for attributes that appertain directly to the capture
declaration, it would be possible to add such support by additionally
allowing for an attribute specifier sequence to precede each capture
declaration.
</p>

<p>
Lambda capture lists are similar to parameter declaration lists but differ in
that implicitly captured entities are not individually declared; there is no
notion of an unnamed capture in the same sense as an unnamed function parameter.
This presents a choice of:
<ol>
  <li>following the general precedent and allowing an attribute specifier
      sequence following the identifier (or <tt>this</tt> keyword) in a simple
      capture or init capture, or</li>
  <li>following the precedent for function parameters and allowing an attribute
      specifier sequence both at the beginning of a capture declaration and
      following the identifier (or <tt>this</tt> keyword).</li>
</ol>
Option 1 is proposed based on the rationale that, since explicitly named
capture declarations always include an identifier (or <tt>this</tt> keyword),
the cognitive and implementation complexity of supporting multiple locations is
not warranted.
</p>

<p>
If it is accepted that an attribute on a lambda capture declaration appertains
to the notional non-static data member, then it follows that an attribute on a
default lambda capture declaration appertains individually to each of the
notional non-static data members of the implicit captures; this is consistent
with attributes specified with function parameter pack declarations.
This could be useful to, for example, apply the <tt>[[no_unique_address]]</tt>
attribute to all implicit captures.
</p>

<p>
Default capture declarations consist solely of either the <tt>&amp;</tt> or
<tt>=</tt> token.
The choices for an attribute specifier sequence are easy; the sequence either
goes before the token, after the token, or at either location.
For consistency with the proposed position for explicit captures, placing the
attribute specifier sequence after the token is proposed.
</p>

<p>
Placing an attribute specifier sequence after <tt>*this</tt> could be a cause
for confusion if programmers interpret it as applying to <tt>this</tt> as
opposed to the intended meaning of it applying to the captured entity, the
object denoted by <tt>*this</tt>.
The remedy for such potential confusion would be to permit an attribute
specifier sequence only at the beginning of each capture declaration.
</p>


<h1 id="proposal">Proposal</h1>

<p>
The following grammar changes are proposed:
<ul>
  <li>For namespace alias definitions:
    <ul>
      <li>Allow an attribute specifier sequence to appear immediately after the
          <tt>namespace</tt> keyword
          (consistent with namespace definitions).</li>
    </ul>
  </li>
  <li>For type template parameters and template template parameters:
    <ul>
      <li>Allow an attribute specifier sequence to appear at the beginning of
          the parameter declaration
          (consistent with non-type template parameter and function parameter
          declarations).</li>
      <li>Allow an attribute specifier sequence to appear following an
          identifier in the parameter declaration
          (consistent with non-type template parameter and function parameter
          declarations).</li>
    </ul>
  </li>
  <li>For lambda captures:
    <ul>
      <li>Allow an attribute specifier sequence to appear following an
          identifier in an explicit capture declaration, but not at the
          beginning of one
          (consistent with the general rule for attribute specifier
          sequences).</li>
      <li>Allow an attribute specifier sequence to appear following
          <tt>this</tt> or <tt>*this</tt> in an explicit capture declaration,
          but not at the beginning of one.</li>
      <li>Allow an attribute specifier sequence to appear following
          <tt>&amp;</tt> or <tt>=</tt> in a capture default declaration,
          but not at the beginning of one.</li>
    </ul>
  </li>
</ul>
</p>

<p>
The following attribute appertainment changes are proposed:
<ul>
  <li>Allow the <tt>[[deprecated]]</tt> attribute to appertain to a namespace
      alias.</li>
  <li>Allow the <tt>[[maybe_unused]]</tt> attribute to appertain to the
      non-static data member, if any, corresponding to a lambda capture.</li>
  <li>Allow the <tt>[[maybe_unused]]</tt> attribute to appertain to type
      template parameters and template template parameters (in addition to
      non-type template parameters).</li>
  <li>Allow the <tt>[[no_unique_address]]</tt> attribute to appertain to the
      non-static data member, if any, corresponding to a lambda capture.</li>
</ul>
</p>

<p>
Taken together, these changes allow for the following:
<blockquote class="code">
namespace ns {}
namespace <font color="green">[[deprecated]]</font> nsa1 = ns; // Ok, previously ill-formed.
namespace nsa2 <font color="red">[[deprecated]]</font> = ns; // Ill-formed; invalid attribute location.

template&lt;<font color="green">[[maybe_unused]]</font> int nttp&gt; // Ok.
void ft1();
template&lt;int nttp <font color="green">[[maybe_unused]]</font>&gt; // Ok.
void ft2();
template&lt;<font color="green">[[maybe_unused]]</font> typename ttp&gt; // Ok, previously ill-formed.
void ft3();
template&lt;typename ttp <font color="green">[[maybe_unused]]</font>&gt; // Ok, previously ill-formed.
void ft4();
template&lt;<font color="green">[[maybe_unused]]</font> template&lt;typename&gt; class tttp&gt; // Ok, previously ill-formed.
void ft5();
template&lt;template&lt;typename&gt; class tttp <font color="green">[[maybe_unused]]</font>&gt; // Ok, previously ill-formed.
void ft6();

struct S {
  void mf(int p) {
    [ <font color="red">[[maybe_unused]]</font> = ]{}();     // Ill-formed; invalid attribute location.
    [ = <font color="green">[[maybe_unused]]</font> ]{}();     // Ok but pointless [[maybe_unused]], previously ill-formed.
    [ <font color="red">[[maybe_unused]]</font> &amp; ]{}();     // Ill-formed; invalid attribute location.
    [ &amp; <font color="green">[[maybe_unused]]</font> ]{}();     // Ok but pointless [[maybe_unused]], previously ill-formed.
    [ <font color="red">[[maybe_unused]]</font> p ]{}();     // Ill-formed; invalid attribute location.
    [ p <font color="green">[[maybe_unused]]</font> ]{}();     // Ok, previously ill-formed.
    [ <font color="red">[[maybe_unused]]</font> this ]{}();  // Ill-formed; invalid attribute location.
    [ this <font color="green">[[maybe_unused]]</font> ]{}();  // Ok, previously ill-formed.
    [ <font color="red">[[maybe_unused]]</font> *this ]{}(); // Ill-formed; invalid attribute location.
    [ *this <font color="green">[[maybe_unused]]</font> ]{}(); // Ok, previously ill-formed.

    [ <font color="red">[[no_unique_address]]</font> = ]{}();     // Ill-formed; invalid attribute location.
    [ = <font color="green">[[no_unique_address]]</font> ]{}();     // Ok, previously ill-formed.
    [ <font color="red">[[no_unique_address]]</font> &amp; ]{}();     // Ill-formed; invalid attribute location.
    [ &amp; <font color="green">[[no_unique_address]]</font> ]{}();     // Ok, previously ill-formed.
    [ <font color="red">[[no_unique_address]]</font> p ]{}();     // Ill-formed; invalid attribute location.
    [ p <font color="green">[[no_unique_address]]</font> ]{}();     // Ok, previously ill-formed.
    [ <font color="red">[[no_unique_address]]</font> this ]{}();  // Ill-formed; invalid attribute location.
    [ this <font color="green">[[no_unique_address]]</font> ]{}();  // Ok, previously ill-formed.
    [ <font color="red">[[no_unique_address]]</font> *this ]{}(); // Ill-formed; invalid attribute location.
    [ *this <font color="green">[[no_unique_address]]</font> ]{}(); // Ok, previously ill-formed.
  }
};
</blockquote>
</p>

<h1 id="implementation-exp">Implementation experience</h1>

<p>
None.
</p>


<h1 id="ack">Acknowledgements</h1>

<p>
Thanks to Aaron Ballman for vetting my expectation that attributes should be
allowed on approximately everything and for providing feedback for multiple
drafts of this proposal.
</p>


<h1 id="references">References</h1>

<table id="references">
  <tr>
    <td id="ref_n3760"><sup>[N3760]</sup></td>
    <td>
      "[[deprecated]] attribute", N3760, 2013.<br/>
      <a href="https://wg21.link/n3760">
      https://wg21.link/n3760</a></td>
  </tr>
  <tr>
    <td id="ref_n4981"><sup>[N4988]</sup></td>
    <td>
      "Working Draft, Standard for Programming Language C++", N4988, 2024.<br/>
      <a href="https://wg21.link/n4988">
      https://wg21.link/n4988</a></td>
  </tr>
  <tr>
    <td id="ref_p0068r0"><sup>[P0068R0]</sup></td>
    <td>
      "Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes.", P0068R0, 2015.<br/>
      <a href="https://wg21.link/p0068r0">
      https://wg21.link/p0068r0</a></td>
  </tr>
  <tr>
    <td id="ref_p0212r1"><sup>[P0212R1]</sup></td>
    <td>
      "Wording for [[maybe_unused]] attribute.", P0212R1, 2016.<br/>
      <a href="https://wg21.link/p0212r1">
      https://wg21.link/p0212r1</a></td>
  </tr>
  <tr>
    <td id="ref_p0840r2"><sup>[P0840R2]</sup></td>
    <td>
      "Language support for empty objects", P0840R2, 2018.<br/>
      <a href="https://wg21.link/p0840r2">
      https://wg21.link/p0840r2</a></td>
  </tr>
</table>


<h1 id="wording">Wording</h1>

<p>
These changes are relative to
<a title="Working Draft, Standard for Programming Language C++"
   href="https://wg21.link/n4988">
N4988</a>
<sup><a title="Working Draft, Standard for Programming Language C++"
        href="#ref_n4988">[N4988]</a></sup>.
</p>

<input type="checkbox" id="hideins">Hide inserted text</input><br/>
<input type="checkbox" id="hidedel">Hide deleted text</input>

<p>
Modify
<a href="http://eel.is/c++draft/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a>:
<blockquote>
  <div style="margin-left: 1em; white-space: nowrap;">
    <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:lambda-capture"><i>lambda-capture</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-default"><i>capture-default</i></a>
      <br/>
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-list"><i>capture-list</i></a>
      <br/>
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-default"><i>capture-default</i></a>
        <tt>,</tt>
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-list"><i>capture-list</i></a>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-default"><i>capture-default</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <tt>&amp;</tt>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
      <br/>
        <tt>=</tt>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-list"><i>capture-list</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture"><i>capture</i></a>
      <br/>
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-list"><i>capture-list</i></a>
        <tt>,</tt>
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture"><i>capture</i></a>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture"><i>capture</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:simple-capture"><i>simple-capture</i></a>
      <br/>
        <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:init-capture"><i>init-capture</i></a>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:simple-capture"><i>simple-capture</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins>
        <tt>...</tt><sub>opt</sub>
      <br/>
        <tt>&amp;</tt>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins>
        <tt>...</tt><sub>opt</sub>
      <br/>
        <tt>this</tt>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
      <br/>
        <tt>*</tt> <tt>this</tt>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:init-capture"><i>init-capture</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <tt>...</tt><sub>opt</sub>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins>
        <a href="http://eel.is/c++draft/dcl.init.general#nt:initializer"><i>initializer</i></a>
      <br/>
        <tt>&amp;</tt>
        <tt>...</tt><sub>opt</sub>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins>
        <a href="http://eel.is/c++draft/dcl.init.general#nt:initializer"><i>initializer</i></a>
      </div>
  </div>
</blockquote>
</p>

<p>
Add an additional paragraph following
<a href="http://eel.is/c++draft/expr.prim.lambda.capture#1">[expr.prim.lambda.capture], paragraph 1</a>:
<blockquote class="stdins">
The optional
<a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a>
in a
<a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:capture-default"><i>capture-default</i></a>
or
<a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:simple-capture"><i>simple-capture</i></a>
or in an
<a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a>
of a
<a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:simple-capture"><i>simple-capture</i></a>
or
<a href="http://eel.is/c++draft/expr.prim.lambda.capture#nt:init-capture"><i>init-capture</i></a>
appertains to the unnamed non-static data member, if any, declared in the
closure type for the captured entity (see below).
</blockquote>
</p>

<p>
Modify
<a href="http://eel.is/c++draft/namespace.alias#1">[namespace.alias], paragraph 1</a>:
<blockquote>
A
<a href="http://eel.is/c++draft/namespace.alias#nt:namespace-alias-definition"><i>namespace-alias-definition</i></a>
declares an alternate name for a namespace according to the following grammar:
<br/><br/>
  <div style="margin-left: 1em; white-space: nowrap;">
    <a href="http://eel.is/c++draft/namespace.alias#nt:namespace-alias"><i>namespace-alias</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/namespace.alias#nt:namespace-alias-definition"><i>namespace-alias-definition</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <tt>namespace</tt>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a>
        <tt>=</tt>
        <a href="http://eel.is/c++draft/namespace.alias#nt:qualified-namespace-specifier"><i>qualified-namespace-specifier</i></a>
        <tt>;</tt>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/namespace.alias#nt:qualified-namespace-specifier"><i>qualified-namespace-specifier</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/expr.prim.id.qual#nt:nested-name-specifier"><i>nested-name-specifier</i></a><sub>opt</sub>
        <a href="http://eel.is/c++draft/namespace.def.general#nt:namespace-name"><i>namespace-name</i></a>
      </div>
  </div>
</blockquote>
</p>

<p>
Add an additional paragraph following
<a href="http://eel.is/c++draft/namespace.alias#2">[namespace.alias], paragraph 2</a>:
<blockquote class="stdins">
The optional
<a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub>
in a
<a href="http://eel.is/c++draft/namespace.alias#nt:namespace-alias-definition"><i>namespace-alias-definition</i></a>
appertains to the namespace alias being defined.
</blockquote>
</p>

<p>
Modify
<a href="http://eel.is/c++draft/dcl.attr.deprecated#2">[dcl.attr.deprecated], paragraph 2</a>:
<blockquote>
The attribute may be applied to the declaration of a class, a
<a href="http://eel.is/c++draft/dcl.typedef#nt:typedef-name">typedef-name</a>,
a variable, a non-static data member, a function, a namespace,
<ins>a namespace alias, </ins>an enumeration,
an enumerator, a concept, or a template specialization.
</blockquote>
</p>

<p>
Modify
<a href="http://eel.is/c++draft/dcl.attr.unused#2">[dcl.attr.unused], paragraph 2</a>:
<blockquote>
The attribute may be applied to the declaration of a class,
<a href="http://eel.is/c++draft/dcl.typedef#nt:typedef-name">typedef-name</a>,
variable (including a structured binding declaration), structured binding,
non-static data member,
<ins>lambda capture, </ins>function,
<ins>function parameter, template parameter, </ins>enumeration, or enumerator,
or to an
<a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a>
label
(<a href="http://eel.is/c++draft/stmt.label">[stmt.label]</a>).
</blockquote>
</p>

<p>
Modify
<a href="http://eel.is/c++draft/dcl.attr.nouniqueaddr#1">[dcl.attr.nouniqueaddr], paragraph 1</a>:
<blockquote>
The
<a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-token"><i>attribute-token</i></a>
<tt>no_unique_address</tt>
specifies that a non-static data member is a potentially-overlapping subobject
(<a href="http://eel.is/c++draft/intro.object">[intro.object]</a>).
No
<a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-argument-clause"><i>attribute-argument-clause</i></a>
shall be present.
The attribute may
<del>appertain</del><ins>be applied</ins>
to a<ins> lambda capture or a</ins>
non-static data member other than a bit-field.
</blockquote>
</p>

<p>
Modify
<a href="http://eel.is/c++draft/temp.param#1">[temp.param], paragraph 1</a>:
<blockquote>
The syntax for
<a href="http://eel.is/c++draft/temp.param#nt:template-parameter"><i>template-parameter</i></a>s
is:
<br/><br/>
  <div style="margin-left: 1em; white-space: nowrap;">
    <a href="http://eel.is/c++draft/temp.param#nt:template-parameter"><i>template-parameter</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/temp.param#nt:type-parameter"><i>type-parameter</i></a>
      <br/>
        <a href="http://eel.is/c++draft/dcl.fct#nt:parameter-declaration"><i>parameter-declaration</i></a>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/temp.param#nt:type-parameter"><i>type-parameter:</i></a>
      <div style="margin-left: 1em; white-space: nowrap;">
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/temp.param#nt:type-parameter-key"><i>type-parameter-key</i></a>
        <tt>...</tt><sub>opt</sub>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins><sub>opt</sub>
      <br/>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/temp.param#nt:type-parameter-key"><i>type-parameter-key</i></a>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins><sub>opt</sub>
        <tt>=</tt>
        <a href="http://eel.is/c++draft/dcl.name#nt:type-id"><i>type-id</i></a>
      <br/>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/temp.param#nt:type-constraint"><i>type-constraint</i></a>
        <tt>...</tt><sub>opt</sub>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins><sub>opt</sub>
      <br/>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/temp.param#nt:type-constraint"><i>type-constraint</i></a>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins><sub>opt</sub>
        <tt>=</tt>
        <a href="http://eel.is/c++draft/dcl.name#nt:type-id"><i>type-id</i></a>
      <br/>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/temp.pre#nt:template-head"><i>template-head</i></a>
        <a href="http://eel.is/c++draft/temp.param#nt:type-parameter-key"><i>type-parameter-key</i></a>
        <tt>...</tt><sub>opt</sub>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins><sub>opt</sub>
      <br/>
        <ins><a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a><sub>opt</sub></ins>
        <a href="http://eel.is/c++draft/temp.pre#nt:template-head"><i>template-head</i></a>
        <a href="http://eel.is/c++draft/temp.param#nt:type-parameter-key"><i>type-parameter-key</i></a>
        <del><a href="http://eel.is/c++draft/lex.name#nt:identifier"><i>identifier</i></a></del><ins><a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a></ins><sub>opt</sub>
        <tt>=</tt>
        <a href="http://eel.is/c++draft/expr.prim.id.general#nt:id-expression"><i>id-expression</i></a>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/temp.param#nt:type-parameter-key"><i>type-parameter-key</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <tt>class</tt>
      <br/>
        <tt>typename</tt>
      </div>
  <br/>
    <a href="http://eel.is/c++draft/temp.param#nt:type-constraint"><i>type-constraint</i></a>:
      <div style="margin-left: 1em; white-space: nowrap;">
        <a href="http://eel.is/c++draft/expr.prim.id.qual#nt:nested-name-specifier"><i>nested-name-specifier</i></a><sub>opt</sub>
        <a href="http://eel.is/c++draft/temp.concept#nt:concept-name"><i>concept-name</i></a>
      <br/>
        <a href="http://eel.is/c++draft/expr.prim.id.qual#nt:nested-name-specifier"><i>nested-name-specifier</i></a><sub>opt</sub>
        <a href="http://eel.is/c++draft/temp.concept#nt:concept-name"><i>concept-name</i></a>
        <tt>&lt;</tt>
        <a href="http://eel.is/c++draft/temp.names#nt:template-argument-list"><i>template-argument-list</i></a><sub>opt</sub>
        <tt>&gt;</tt>
      </div>
  </div>
<br/>
The component names of a
<a href="http://eel.is/c++draft/temp.param#nt:type-constraint"><i>type-constraint</i></a>
are its
<a href="http://eel.is/c++draft/temp.concept#nt:concept-name"><i>concept-name</i></a>
and those of its
<a href="http://eel.is/c++draft/expr.prim.id.qual#nt:nested-name-specifier"><i>nested-name-specifier</i></a>
(if any).
<br/><br/>
[<em>Note 1</em>: The <tt>&gt;</tt> token following the
<a href="http://eel.is/c++draft/temp.pre#nt:template-parameter-list"><i>template-parameter-list</i></a>
of a
<a href="http://eel.is/c++draft/temp.param#nt:type-parameter"><i>type-parameter</i></a>
can be the product of replacing a <tt>&gt;&gt;</tt> token by two consecutive
<tt>&gt;</tt> tokens
(<a href="http://eel.is/c++draft/temp.names">[temp.names]</a>).
&mdash; <em>end note</em>]
</blockquote>
</p>

<p>
Add an additional paragraph following
<a href="http://eel.is/c++draft/temp.param#2">[temp.param], paragraph 2</a>:
<blockquote class="stdins">
The optional
<a href="http://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq"><i>attribute-specifier-seq</i></a>
in a
<a href="http://eel.is/c++draft/temp.param#nt:type-parameter"><i>type-parameter</i></a>
or in an
<a href="http://eel.is/c++draft/dcl.pre#nt:attributed-identifier"><i>attributed-identifier</i></a>
of a
<a href="http://eel.is/c++draft/temp.param#nt:type-parameter"><i>type-parameter</i></a>
appertains to the type parameter being declared.
</blockquote>
</p>

<p>
Modify Annex A, "Grammar summary",
<a href="http://eel.is/c++draft/gram">[gram]</a>
to reflect the grammar changes to
<a href="http://eel.is/c++draft/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a>,
<a href="http://eel.is/c++draft/namespace.alias">[namespace.alias]</a>,
and
<a href="http://eel.is/c++draft/temp.param">[temp.param]</a>
shown above.
</p>


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