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<title>Issue 2782: scoped_allocator_adaptor constructors must be constrained</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++17">C++17</a> status.</em></p>
<h3 id="2782"><a href="lwg-defects.html#2782">2782</a>. <code>scoped_allocator_adaptor</code> constructors must be constrained</h3>
<p><b>Section:</b> 20.6.3 <a href="https://wg21.link/allocator.adaptor.cnstr">[allocator.adaptor.cnstr]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2016-10-14 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The templated constructors of <code>scoped_allocator_adaptor</code> need to be constrained, otherwise uses-allocator 
construction gives the wrong answer and causes errors for code that should compile.
<p/>
Consider two incompatible allocator types:
</p>
<blockquote><pre>
template&lt;class T&gt; struct Alloc1 { ... };
template&lt;class T&gt; struct Alloc2 { ... };
static_assert(!is_convertible_v&lt;Alloc1&lt;int&gt;, Alloc2&lt;int&gt;&gt;);
</pre></blockquote>
<p>
The unconstrained constructors give this bogus answer:
</p>
<blockquote><pre>
template&lt;class T&gt; using scoped = scoped_allocator_adaptor&lt;T&gt;;
static_assert(is_convertible_v&lt;scoped&lt;Alloc1&lt;int&gt;&gt;, scoped&lt;Alloc2&lt;int&gt;&gt;&gt;);
</pre></blockquote>
<p>
This causes <code>uses_allocator</code> to give the wrong answer for any specialization involving incompatible 
<code>scoped_allocator_adaptors</code>, which makes <code>scoped_allocator_adaptor::construct()</code> take an ill-formed 
branch e.g.
</p>
<blockquote><pre>
struct X 
{
  using allocator_type = scoped&lt;Alloc2&lt;int&gt;&gt;;
  X(const allocator_type&amp;);
  X();
};
scoped&lt;Alloc1&lt;int&gt;&gt;{}.construct((X*)0);
</pre></blockquote>
<p>
This fails to compile, because <code>uses_allocator&lt;X, scoped_allocator_adaptor&lt;Alloc2&lt;int&gt;&gt;&gt;</code> is 
<code>true</code>, so the allocator is passed to the <code>X</code> constructor, but the conversion fails. The error is outside 
the immediate context, and so is a hard error.
</p>

<p><i>[2016-11-12, Issaquah]</i></p>

<p>Sat AM: Priority 0; move to Ready</p>
<p>Billy to open another issue about the confusion with the ctor</p>


<p id="res-2782"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4606.
</p>

<ol>
<li><p>Modify 20.6.3 <a href="https://wg21.link/allocator.adaptor.cnstr">[allocator.adaptor.cnstr]</a> by converting "Requires" elements to "Remarks: shall not participate ..." constraints as shown:</p>

<blockquote>
<pre>
template &lt;class OuterA2&gt;
  scoped_allocator_adaptor(OuterA2&amp;&amp; outerAlloc,
                           const InnerAllocs&amp;... innerAllocs) noexcept;
</pre>
<blockquote>
<p>
<del>-2- <i>Requires:</i> <code>OuterAlloc</code> shall be constructible from <code>OuterA2</code>.</del>
<p/>
-3- <i>Effects:</i> Initializes the <code>OuterAlloc</code> base class with <code>std::forward&lt;OuterA2&gt;(outerAlloc)</code> and 
inner with <code>innerAllocs...</code> (hence recursively initializing each allocator within the adaptor with the
corresponding allocator from the argument list).
<p/>
<ins>-?- <i>Remarks:</i> This constructor shall not participate in overload resolution unless 
<code>is_constructible_v&lt;OuterAlloc, OuterA2&gt;</code> is <code>true</code>.</ins>
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class OuterA2&gt;
  scoped_allocator_adaptor(const scoped_allocator_adaptor&lt;OuterA2,
                           InnerAllocs...&gt;&amp; other) noexcept;
</pre>
<blockquote>
<p>
<del>-6- <i>Requires:</i> <code>OuterAlloc</code> shall be constructible from <code>OuterA2</code>.</del>
<p/>
-7- <i>Effects:</i> Initializes each allocator within the adaptor with the corresponding allocator from <code>other</code>.
<p/>
<ins>-?- <i>Remarks:</i> This constructor shall not participate in overload resolution unless 
<code>is_constructible_v&lt;OuterAlloc, const OuterA2&amp;&gt;</code> is <code>true</code>.</ins>
</p>
</blockquote>
<pre>
template &lt;class OuterA2&gt;
  scoped_allocator_adaptor(scoped_allocator_adaptor&lt;OuterA2,
                           InnerAllocs...&gt;&amp;&amp; other) noexcept;
</pre>
<blockquote>
<p>
<del>-8- <i>Requires:</i> <code>OuterAlloc</code> shall be constructible from <code>OuterA2</code>.</del>
<p/>
-9- <i>Effects:</i> Initializes each allocator within the adaptor with the corresponding allocator rvalue from <code>other</code>.
<p/>
<ins>-?- <i>Remarks:</i> This constructor shall not participate in overload resolution unless 
<code>is_constructible_v&lt;OuterAlloc, OuterA2&gt;</code> is <code>true</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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