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        Structured binding declaration as a condition - HackMD
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<tr><th>Doc. no.:</th>    <td>P0963R0</td></tr>
<tr><th>Date:</th>    <td>2018-02-05</td></tr>
<tr><th>Audience:</th>    <td>Evolution Working Group</td></tr>
<tr><th>Reply-to:</th>    <td>Zhihao Yuan &lt;zy at miator dot net&gt;</td></tr>
</tbody></table><h1 id="Structured-binding-declaration-as-a-condition"><a title="Structured-binding-declaration-as-a-condition" class="anchor hidden-xs" href="#Structured-binding-declaration-as-a-condition"><span class="octicon octicon-link"></span></a>Structured binding declaration as a <em>condition</em></h1><h2 id="Motivation"><a title="Motivation" class="anchor hidden-xs" href="#Motivation"><span class="octicon octicon-link"></span></a>Motivation</h2><pre><code class="c++ hljs"><span class="hljs-keyword">if</span> (<span class="hljs-keyword">auto</span> [ok, value] = readint(...); ok == format_errc::no_error)
</code></pre><p>is not as clear and simple as</p><pre><code class="c++ hljs"><span class="hljs-keyword">if</span> (<span class="hljs-keyword">auto</span> [ok, value] = readint(...))
</code></pre><p>when</p><pre><code class="c++ hljs"><span class="hljs-keyword">if</span> (readint(...))
</code></pre><p>is starting a valid <code>if</code> statement.</p><h2 id="Discussion"><a title="Discussion" class="anchor hidden-xs" href="#Discussion"><span class="octicon octicon-link"></span></a>Discussion</h2><p>C++17 structured binding declaration is designed as a variant of variable declarations.  More specifically, it is a variant to <em>simple-declaration</em> and <em>for-range-declaration</em>.  However, the grammar production <em>condition</em> is, conditionally, also a kind of variable declaration,</p><pre><code class="c++ hljs"><span class="hljs-keyword">if</span> (<span class="hljs-keyword">auto</span> rc = readint(...))
</code></pre><p>so there is little reason not to allow the above to be replaced by</p><pre><code class="c++ hljs"><span class="hljs-keyword">if</span> (<span class="hljs-keyword">auto</span> [ok, value] = readint(...))
</code></pre><p>when the return type qualifies contextually convertible to bool and decomposable at the same time.</p><p>In a word, we consider the status quo as a wording oversight, presumably affected by the fact that there is no BNF production named “condition-declaration” in the standard text.  Therefore we suggest applying the proposed change as a DR against C++17.</p><h2 id="Design-Decisions"><a title="Design-Decisions" class="anchor hidden-xs" href="#Design-Decisions"><span class="octicon octicon-link"></span></a>Design Decisions</h2><p>It is tempting to add extra semantics given the proposed syntax, such as making the decomposition work (accessing <code>e.get()</code>, for example) conditional when the underlying object for decomposition, <code>e</code>, evaluates to <code>false</code> after contextually converted to <code>bool</code>.  It may enable more programming styles such as throwing <code>get()</code>.  The author considers that this idea complicates the mental model for using the two features together, and decided not to include it in this paper.</p><p>It is worthwhile to figure out what array decomposition does in <em>condition</em>.  <em>condition</em> bans declaring arrays, so this paper does not allow decomposing array <code>e</code> either.  However, <em>condition</em> accepts array references, which always evaluate to <code>true</code>, and this is also unchanged in this paper.  That is,</p><pre><code class="c++ hljs"><span class="hljs-keyword">if</span> (<span class="hljs-keyword">auto</span>&amp; [a, b, c] = <span class="hljs-string">"ht"</span>)
</code></pre><p>works with the proposed change.</p><p>Technically we can enable decomposing arrays in <em>conditions</em>, but the author is not motivated to evaluate.</p><h2 id="Wording"><a title="Wording" class="anchor hidden-xs" href="#Wording"><span class="octicon octicon-link"></span></a>Wording</h2><p>The wording is relative to N4713.</p><p>Extend the grammar in 9 [stmt.stmt]/1 as follows:</p><blockquote>
<p><em>condition:</em><br>
&nbsp;&nbsp;&nbsp;<em>expression</em><br>
&nbsp;&nbsp;&nbsp;<em>attribute-specifier-seq<sub>opt</sub> decl-specifier-seq declarator brace-or-equal-initializer</em><br>
&nbsp;&nbsp;&nbsp;<ins><em>attribute-specifier-seq<sub>opt</sub> decl-specifier-seq ref-qualifier<sub>opt</sub></em> <code>[</code> <em>identifier-list</em> <code>]</code> <em>brace-or-equal-initializer</em></ins></p>
</blockquote><p>Modify 9 [stmt.stmt]/2 as follows:</p><blockquote>
<p>The rules for <em>conditions</em> apply both to selection-statements and to the <code>for</code> and <code>while</code> statements (9.5).
The <em>declarator</em> shall not specify a function or an array. The <em>decl-specifier-seq</em> shall not define a class or
enumeration. If the <code>auto</code> <em>type-specifier</em> appears in the <em>decl-specifier-seq</em>, the type of the identifier being
declared is deduced from the initializer as described in 10.1.7.4. <ins>If <em>identifier-list</em> appears in the <em>condition</em>, the declaration is a structured binding declaration (11.5), where the <em>assignment-expression</em> in the <em>brace-or-equal-initializer</em> shall not have array type if no <em>ref-qualifier</em> is present.</ins></p>
</blockquote><p>Modify paragraph 3 as follows:</p><blockquote>
<p>A name introduced by a declaration in a <em>condition</em> (<s>either</s><ins>may be</ins> introduced by the <em>decl-specifier-seq</em><ins>,</ins> <s>or</s>the <em>declarator</em><ins>, or the <em>identifier-list</em></ins> of the condition) is in scope from its point of declaration until the end of the substatements controlled by the condition. If the name is redeclared in the outermost block of a substatement controlled by the condition, the declaration that redeclares the name is ill-formed. <em>[ Example:</em></p>
<pre><code>if (int x = f()) {
  int x;            // ill-formed, redeclaration of x
}
else {
  int x;            // ill-formed, redeclaration of x
}
</code></pre>
<pre><ins>if (auto&amp; [a] = "") {
  char a;           // ill-formed, redeclaration of a
}
</ins></pre>
<p><em>–end example ]</em></p>
</blockquote><p>Insert a paragraph between paragraph 3 and 4 in 9 [stmt.stmt]:</p><blockquote>
<p>The variable of a <em>condition</em> that is an initialized declaration is the declared variable.  The variable of a <em>condition</em> that is a structured binding declaration (11.5) is the variable <code>e</code> with
a unique name.</p>
</blockquote><p>Rewrite the original paragraph 4 as follows:</p><blockquote>
<p>If a <em>condition</em> is an expression, the value of the condition is the value of the expression, contextually converted to <code>bool</code> for statements other than <code>switch</code>; if that conversion is ill-formed,
the program is ill-formed.  Otherwise, in a <code>switch</code> statement, the value of the condition is the value of the variable of the <em>condition</em> if it has integral or enumeration type, or of that variable implicitly converted to integral or enumeration type otherwise.  In a statement other than a <code>switch</code> statement, the value of the condition is the value of the variable of the <em>condition</em> contextually converted to <code>bool</code> (Clause 7).  If that conversion is ill-formed, the program is ill-formed.  The value of the condition will be referred to as simply “the condition” where the usage is unambiguous.</p>
</blockquote><h2 id="Implementation"><a title="Implementation" class="anchor hidden-xs" href="#Implementation"><span class="octicon octicon-link"></span></a>Implementation</h2><p>The proposed change has been implemented in Clang 6.0.0 guarded by <code>-Wbinding-in-condition</code>: <a href="https://godbolt.org/g/hgefUz" target="_blank">https://godbolt.org/g/hgefUz</a></p><p>The implementation is a one-line-change without counting in diagnosis and tests, giving a sense of how natural the proposed change is.</p><h2 id="Ackownledgement"><a title="Ackownledgement" class="anchor hidden-xs" href="#Ackownledgement"><span class="octicon octicon-link"></span></a>Ackownledgement</h2><p>Thank Richard Smith for encouraging the work.</p></div>
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