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            <article class="post">
                <p>
                    Document number: P0077R0<br>
                    Supersedes: N4446<br>
                    Date: 2015-09-22<br>
                    Project: Programming Language C++, Library Evolution Working Group<br>
                    Reply-to: Agustín Bergé <a href="mailto:agustinberge@gmail.com">agustinberge@gmail.com</a>
                </p>
                <a name="is_callable-the-missing-_`invoke`_-related-trait" href="#is_callable-the-missing-_`invoke`_-related-trait">
                    <h1><code>is_callable</code>, the missing <em><code>INVOKE</code></em> related trait</h1>
                </a>
                <a name="0.-history" href="#0.-history"><h2>0. History</h2></a>
                <ul>
                    <li>
                        <p>Add discussion on alternative syntax.</p>
                    </li>
                    <li>
                        <p>Add discussion on additional nothrow trait, add <code>is_nothrow_callable</code>.</p>
                    </li>
                    <li>
                        <p>Add feature-testing macro recomendation.</p>
                    </li>
                    <li>
                        <p>Remove discussion on naming.</p>
                    </li>
                </ul>
                <a name="1.-introduction" href="#1.-introduction"><h2>1. Introduction</h2></a>
                <p>
                    This paper proposes to introduce a new trait to determine whether an
                    <em><code>INVOKE</code></em> expression is well formed.
                </p>
                <a name="2.-motivation" href="#2.-motivation"><h2>2. Motivation</h2></a>
                <p>
                    Starting with C++11, the library introduced the pseudo-macro <em><code>INVOKE</code></em> as a
                    way to uniformly handle function objects and member pointers as call
                    expressions. The trait <code>result_of</code> was made to follow <em><code>INVOKE</code></em> semantics
                    as well. This left users —who want to follow the precedence set forth by
                    the standard library— with the correct result type but no direct way of
                    obtaining such result, and <code>invoke</code> implementations proliferated.
                </p>
                <p>
                    This was recently rectified by the introduction of <code>invoke</code> to the working
                    draft [N4169]. However, there is still one piece of the puzzle missing, and is
                    the ability to query whether an <em><code>INVOKE</code></em> expression is well formed when
                    treated as an unevaluated operand. Such functionality is currently present in
                    the form of C++14 SFINAE-friendly <code>result_of</code>, albeit in a non user-friendly
                    way, and it should be made readily available in trait form for the same
                    reasons <code>invoke</code> was introduced into the library.
                </p>
                <p>The following is an artist depiction of such trait:</p>
                <div>
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                                            <div class="line number1 index0 alt2"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">T, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R = </code><code class="cpp keyword bold">void</code><code class="cpp plain">, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">= </code><code class="cpp keyword bold">void</code><code class="cpp plain">&gt;</code></div>
                                            <div class="line number2 index1 alt1"><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable</code></div>
                                            <div class="line number3 index2 alt2"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp plain">: false_type</code></div>
                                            <div class="line number4 index3 alt1"><code class="cpp plain">{};</code></div>
                                            <div class="line number5 index4 alt2">&nbsp;</div>
                                            <div class="line number6 index5 alt1"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">T&gt;</code></div>
                                            <div class="line number7 index6 alt2"><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable&lt;T, </code><code class="cpp keyword bold">void</code><code class="cpp plain">, void_t&lt;result_of_t&lt;T&gt;&gt;&gt;</code></div>
                                            <div class="line number8 index7 alt1"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp plain">: true_type</code></div>
                                            <div class="line number9 index8 alt2"><code class="cpp plain">{};</code></div>
                                            <div class="line number10 index9 alt1">&nbsp;</div>
                                            <div class="line number11 index10 alt2"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">T, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R&gt;</code></div>
                                            <div class="line number12 index11 alt1"><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable&lt;T, R, void_t&lt;result_of_t&lt;T&gt;&gt;&gt;</code></div>
                                            <div class="line number13 index12 alt2"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp plain">: is_convertible&lt;result_of_t&lt;T&gt;, R&gt;</code></div>
                                            <div class="line number14 index13 alt1"><code class="cpp plain">{};</code></div>
                                        </div>
                                    </td>
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                    </div>
                </div>
                <p>
                    This trait is implemented in the wild under different names, and the check for
                    a compatible result type is not always present. <a href="http://talesofcpp.fusionfenix.com/post-11/true-story-call-me-maybe" title="True Story: Call Me Maybe - Tales of C++">This post</a> [call-me-maybe]
                    shows how the implementation of such trait has been both improved and
                    simplified by every new standard.
                </p>
                <a name="3.-design-questions" href="#3.-design-questions"><h2>3. Design questions</h2></a>
                <a name="3.1-compatible-return-types" href="#3.1-compatible-return-types">
                    <h3>3.1 Compatible return types</h3>
                </a>
                <p>
                    <em><code>INVOKE</code></em> comes in two flavors, the primary <code>INVOKE(f, t1, t2, ..., tN)</code>
                    and <code>INVOKE(f, t1, t2, ..., tN, R)</code> defined as <code>INVOKE(f, t1, t2, ..., tN)</code>
                    implicitly converted to <code>R</code>. After the resolution of <a href="http://cplusplus.github.io/LWG/lwg-defects.html#2420">LWG2420</a>, <code>
                        INVOKE(f, t1, t2, ...,
                        tN, void)
                    </code> is defined as <code>static_cast&lt;void&gt;(INVOKE(f, t1, t2, ..., tN))</code>. Both
                    flavors can be supported with a defaulted template argument:
                </p>
                <div>
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                                            <div class="line number1 index0 alt2"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code><code class="cpp plain">, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R = </code><code class="cpp keyword bold">void</code><code class="cpp plain">&gt; </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable; </code><code class="cpp comments">// not defined</code></div>
                                            <div class="line number2 index1 alt1"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R&gt;</code></div>
                                            <div class="line number3 index2 alt2"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable&lt;Fn(ArgTypes...), R&gt;;</code></div>
                                        </div>
                                    </td>
                                </tr>
                            </tbody>
                        </table>
                    </div>
                </div>
                <p>
                    However, if only one of those flavors would be supported there would be no
                    missing functionality, only more work for the user.
                </p>
                <a name="3.2-alternative-parameter-syntax" href="#3.2-alternative-parameter-syntax">
                    <h3>3.2 Alternative parameter syntax</h3>
                </a>
                <blockquote>
                    <p>Someone suggests alternative parameter syntax, <code>is_callable&lt;Fn, R(Args...)&gt;</code>.</p>
                    <p>Do we want AB to add consideration of the alternative syntax in the paper?</p>
                    <table>
                        <thead>
                            <tr>
                                <th align="center">SF</th>
                                <th align="center">F</th>
                                <th align="center">N</th>
                                <th align="center">A</th>
                                <th align="center">SA</th>
                            </tr>
                        </thead>
                        <tbody>
                            <tr>
                                <td align="center">0</td>
                                <td align="center">6</td>
                                <td align="center">5</td>
                                <td align="center">0</td>
                                <td align="center">0</td>
                            </tr>
                        </tbody>
                    </table>
                    <p>
                        But consistency between this and invocation_traits (in Fundamentals v1) is
                        important.
                    </p>
                </blockquote>
                <p>
                    The syntax used by the invocation traits in the Library Fundamentals TS is
                    <code>Fn(Args...)</code>, which is consistent with the syntax used by <code>std::result_of</code>.
                    The optional trailing <code>R</code> for a checked compatible return type is consistent
                    with the alternative flavor of <em><code>INVOKE</code></em>.
                </p>
                <div>
                    <div id="highlighter_576089" class="syntaxhighlighter nogutter  cpp">
                        <table border="0" cellpadding="0" cellspacing="0">
                            <tbody>
                                <tr>
                                    <td class="code">
                                        <div class="container">
                                            <div class="line number1 index0 alt2"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes&gt;</code></div>
                                            <div class="line number2 index1 alt1"><code class="cpp keyword bold">struct</code> <code class="cpp plain">invocation_type&lt;Fn(ArgTypes...)&gt;;</code></div>
                                            <div class="line number3 index2 alt2">&nbsp;</div>
                                            <div class="line number4 index3 alt1"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes&gt;</code></div>
                                            <div class="line number5 index4 alt2"><code class="cpp keyword bold">struct</code> <code class="cpp plain">result_of&lt;Fn(ArgTypes...)&gt;;</code></div>
                                        </div>
                                    </td>
                                </tr>
                            </tbody>
                        </table>
                    </div>
                </div>
                <p>
                    For consistency with the rest of the standard library, the suggested syntax is
                    <code>Fn(ArgTypes...)</code> for representing an instance of a callable <code>Fn</code> invoked with
                    arguments of type <code>ArgTypes...</code>.
                </p>
                <a name="3.3-additional-nothrow-trait" href="#3.3-additional-nothrow-trait">
                    <h3>3.3 Additional nothrow trait</h3>
                </a>
                <blockquote>
                    <p>Add <code>is_noexcept_callable</code>?</p>
                    <table>
                        <thead>
                            <tr>
                                <th align="center">SF</th>
                                <th align="center">F</th>
                                <th align="center">N</th>
                                <th align="center">A</th>
                                <th align="center">SA</th>
                            </tr>
                        </thead>
                        <tbody>
                            <tr>
                                <td align="center">1</td>
                                <td align="center">4</td>
                                <td align="center">4</td>
                                <td align="center">2</td>
                                <td align="center">0</td>
                            </tr>
                        </tbody>
                    </table>
                </blockquote>
                <p>
                    Traits that check whether certain expressions involving special member
                    functions are well-formed also ship an additional <code>nothrow</code> trait, that
                    reports the result of applying the <code>noexcept</code> operator to the expression. It
                    is reasonable to provide a similar additional <code>nothrow</code> trait for
                    <code>is_callable</code>, <code>is_nothrow_callable</code>, that reports whether the given
                    <em><code>INVOKE</code></em> expression is known not to throw any exceptions.
                </p>
                <p>
                    It should be noted that the standard library does not specify an exception
                    specification for its callable types (like <code>reference_wrapper</code>), but a
                    conforming implementation may add a non-throwing noexcept-specification.
                    The result of <code>is_nothrow_callable</code> when a standard library callable type is
                    involved is thus implementation defined.
                </p>
                <a name="4.-feature-testing" href="#4.-feature-testing"><h2>4. Feature-testing</h2></a>
                <p>
                    For the purposes of SG10, we recommend a feature-testing macro named
                    <code>__cpp_lib_experimental_is_callable</code>.
                </p>
                <a name="5.-proposed-wording" href="#5.-proposed-wording"><h2>5. Proposed Wording</h2></a>
                <p>This wording is relative to [N4527].</p>
                <p>
                    Change 20.10.2 [meta.type.synop], header <code>&lt;type_traits&gt;</code> synopsis, as
                    indicated
                </p>
                <blockquote>
                    <div>
                        <div id="highlighter_928120" class="syntaxhighlighter nogutter  cpp">
                            <table border="0" cellpadding="0" cellspacing="0">
                                <tbody>
                                    <tr>
                                        <td class="code">
                                            <div class="container">
                                                <div class="line number1 index0 alt2"><code class="cpp keyword bold">namespace</code> <code class="cpp plain">std {</code></div>
                                                <div class="line number2 index1 alt1"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><code class="cpp plain">[...]</code></div>
                                                <div class="line number3 index2 alt2"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><code class="cpp comments">// 20.10.4.3, type properties:</code></div>
                                                <div class="line number4 index3 alt1"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><code class="cpp plain">[...]</code></div>
                                                <div class="line number5 index4 alt2"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">T&gt; </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_nothrow_destructible;</code></div>
                                                <div class="line number6 index5 alt1"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">T&gt; </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">has_virtual_destructor;</code></div>
                                                <div class="line number7 index6 alt2"><code class="cpp spaces">&nbsp;&nbsp;</code>&nbsp;</div>
                                                <div class="line number8 index7 alt1"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><ins style="background-color: #A0FFA0;"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code><code class="cpp plain">, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R = </code><code class="cpp keyword bold">void</code><code class="cpp plain">&gt; </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable; </code><code class="cpp comments">// not defined</code></ins></div>
                                                <div class="line number9 index8 alt2"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><ins style="background-color: #A0FFA0;"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R&gt;</code></ins></div>
                                                <div class="line number10 index9 alt1"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><ins style="background-color: #A0FFA0;"><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable&lt;Fn(ArgTypes...), R&gt;;</code></ins></div>
                                                <div class="line number11 index10 alt2"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><ins style="background-color: #A0FFA0;"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code><code class="cpp plain">, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R = </code><code class="cpp keyword bold">void</code><code class="cpp plain">&gt; </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_nothrow_callable; </code><code class="cpp comments">// not defined</code></ins></div>
                                                <div class="line number12 index11 alt1"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><ins style="background-color: #A0FFA0;"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R&gt;</code></ins></div>
                                                <div class="line number13 index12 alt2"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><ins style="background-color: #A0FFA0;"><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_nothrow_callable&lt;Fn(ArgTypes...), R&gt;;</code></div>
                                                <div class="line number14 index13 alt1"><code class="cpp spaces">&nbsp;&nbsp;</code>&nbsp;</div>
                                                <div class="line number15 index14 alt2"><code class="cpp spaces">&nbsp;&nbsp;&nbsp;&nbsp;</code><code class="cpp plain">[...]</code></div>
                                                <div class="line number16 index15 alt1"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp plain">}</code></div>
                                            </div>
                                        </td>
                                    </tr>
                                </tbody>
                            </table>
                        </div>
                    </div>
                </blockquote>
                <p>
                    Change 20.10.4.3 [meta.unary.prop], Table 49 — Type property predicates, add
                    new rows with the following content:
                </p>
                <p><strong>Template</strong>:</p>
                <blockquote>
                    <div>
                        <div id="highlighter_891630" class="syntaxhighlighter nogutter  cpp">
                            <table border="0" cellpadding="0" cellspacing="0">
                                <tbody>
                                    <tr>
                                        <td class="code">
                                            <div class="container">
                                                <div class="line number1 index0 alt2"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R&gt;</code></div>
                                                <div class="line number2 index1 alt1"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_callable&lt;Fn(ArgTypes...), R&gt;;</code></div>
                                            </div>
                                        </td>
                                    </tr>
                                </tbody>
                            </table>
                        </div>
                    </div>
                </blockquote>
                <p><strong>Condition</strong>:</p>
                <blockquote>
                    <p>
                        The expression <code>INVOKE(declval&lt;Fn&gt;(), declval&lt;ArgTypes&gt;()..., R)</code> is well
                        formed when treated as an unevaluated operand.
                    </p>
                </blockquote>
                <p><strong>Preconditions</strong>:</p>
                <blockquote>
                    <p>
                        <code>Fn</code> and all types in the parameter pack <code>ArgTypes</code> shall be complete types,
                        (possibly cv-qualified) <code>void</code>, or arrays of unknown bound.
                    </p>
                </blockquote>
                <p><strong>Template</strong>:</p>
                <blockquote>
                    <div>
                        <div id="highlighter_820154" class="syntaxhighlighter nogutter  cpp">
                            <table border="0" cellpadding="0" cellspacing="0">
                                <tbody>
                                    <tr>
                                        <td class="code">
                                            <div class="container">
                                                <div class="line number1 index0 alt2"><code class="cpp keyword bold">template</code> <code class="cpp plain">&lt;</code><code class="cpp keyword bold">class</code> <code class="cpp plain">Fn, </code><code class="cpp keyword bold">class</code><code class="cpp plain">... ArgTypes, </code><code class="cpp keyword bold">class</code> <code class="cpp plain">R&gt;</code></div>
                                                <div class="line number2 index1 alt1"><code class="cpp spaces">&nbsp;&nbsp;</code><code class="cpp keyword bold">struct</code> <code class="cpp plain">is_nothrow_callable&lt;Fn(ArgTypes...), R&gt;;</code></div>
                                            </div>
                                        </td>
                                    </tr>
                                </tbody>
                            </table>
                        </div>
                    </div>
                </blockquote>
                <p><strong>Condition</strong>:</p>
                <blockquote>
                    <p>
                        <code>is_callable&lt;Fn(ArgTypes...), R&gt;::value</code> is <code>true</code> and the expression
                        <code>INVOKE(declval&lt;Fn&gt;(), declval&lt;ArgTypes&gt;()..., R)</code> is known not to throw any
                        exception.
                    </p>
                </blockquote>
                <p><strong>Preconditions</strong>:</p>
                <blockquote>
                    <p>
                        <code>Fn</code> and all types in the parameter pack <code>ArgTypes</code> shall be complete types,
                        (possibly cv-qualified) <code>void</code>, or arrays of unknown bound.
                    </p>
                </blockquote>
                <a name="6.-references" href="#6.-references"><h2>6. References</h2></a>
                <ul>
                    <li>
                        <p>
                            [N4527] ISO/IEC JTC1 SC22 WG21, Programming Languages - C++, working draft, May 2015<br>
                            <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4527.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4527.pdf</a>
                        </p>
                    </li>
                    <li>
                        <p>
                            [call-me-maybe] True Story: Call Me Maybe - Tales of C++<br>
                            <a href="http://talesofcpp.fusionfenix.com/post-11/true-story-call-me-maybe">http://talesofcpp.fusionfenix.com/post-11/true-story-call-me-maybe</a>
                        </p>
                    </li>
                    <li>
                        <p>
                            [N4169] A proposal to add invoke function template (Revision 1) - Tomasz Kaminski<br>
                            <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4169.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4169.html</a>
                        </p>
                    </li>
                </ul>
            </article>
        </section>
    </div>
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