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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-43041] DefaultHash: avoid using Abseil internals</title>
                <link>https://jira.mongodb.org/browse/SERVER-43041</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description></description>
                <environment></environment>
        <key id="906958">SERVER-43041</key>
            <summary>DefaultHash: avoid using Abseil internals</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="2">Won&apos;t Fix</resolution>
                                        <assignee username="backlog-server-servicearch">Backlog - Service Architecture</assignee>
                                    <reporter username="billy.donahue@mongodb.com">Billy Donahue</reporter>
                        <labels>
                    </labels>
                <created>Mon, 26 Aug 2019 17:45:28 +0000</created>
                <updated>Tue, 6 Dec 2022 02:49:39 +0000</updated>
                            <resolved>Thu, 13 Aug 2020 14:44:16 +0000</resolved>
                                                                    <component>Internal Code</component>
                                        <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="2387366" author="billy.donahue" created="Mon, 26 Aug 2019 21:28:30 +0000"  >&lt;p&gt;Difficult.&lt;/p&gt;

&lt;p&gt;I thought we&apos;d just make an&#160;absl::node_hash_set&amp;lt;K&amp;gt; and extract its hasher, relying on substitution failure to catch K for which there is no default hasher.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://mongodbcr.appspot.com/510430012/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://mongodbcr.appspot.com/510430012/&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;But no, that doesn&apos;t work because of some unusual absl internals.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/def30acb316289d5cb95ecdf2654c3ca919c6a46/src/third_party/abseil-cpp-master/abseil-cpp/absl/container/internal/raw_hash_set.h#L677&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/def30acb316289d5cb95ecdf2654c3ca919c6a46/src/third_party/abseil-cpp-master/abseil-cpp/absl/container/internal/raw_hash_set.h#L677&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;This appears inside the raw_hash_set class template:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// Give an early error when key_type is not hashable/eq. &lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;auto KeyTypeCanBeHashed(const Hash&amp;amp; h, const key_type&amp;amp; k) -&amp;gt; decltype(h(k)); &lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;The particular -&amp;gt;decltype(h(k)) syntax makes it impossible to make the container for a key that doesn&apos;t have a default hasher and rely on sfinae to catch the cases where a default hasher can&apos;t be determined.&#160; Small repro:&#160;&lt;a href=&quot;https://godbolt.org/z/-XCK-O&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://godbolt.org/z/-XCK-O&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;We might be able to convince them to make the containers more sfinae friendly, or to make the default_hasher metafunction public.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;We can also just make our OWN metafunction that determines default hasher and use that for our stdx::unordered_{map,set} containers.&lt;/p&gt;

&lt;p&gt;In the case that our metafunction can&apos;t find a hasher, we fall back to absl&apos;s hasher choices, and if that fails, well, we tried.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;</comment>
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