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<title>Issue 3524: Unimplementable narrowing and evaluation order requirements for range adaptors</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#Resolved">Resolved</a> status.</em></p>
<h3 id="3524"><a href="lwg-defects.html#3524">3524</a>. Unimplementable narrowing and evaluation order requirements for range adaptors</h3>
<p><b>Section:</b> 25.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-02-19 <b>Last modified:</b> 2021-06-14</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The specification of various range factory and adaptor objects generally says that some function call 
expression on them is expression-equivalent to an expression that performs list-initialization or in 
some cases a comma expression. This imposes evaluation order requirements that are unlikely to be 
intended and sometimes outright contradictory. For example, 25.7.12.1 <a href="https://wg21.link/range.drop.overview">[range.drop.overview]</a> says 
that <code>views::drop(E, F)</code> is expression-equivalent to "<code>((void) F, decay-copy(E))</code>" in 
one case, and <code>drop_view{E, F}</code> in another. The first expression requires <code>F</code> to be 
sequenced before <code>E</code>, while the second expression requires <code>E</code> to be sequenced before 
<code>F</code>. They can't both hold in the absence of high levels of compiler magic.
<p/>
Additionally, because the core language narrowing check in list-initialization considers the value of 
constant expressions, "expression-equivalent" also requires the constantness to be propagated. For 
example, given a range <code>E</code> whose <code>difference_type</code> is <code>int32_t</code>, 
<code>views::drop(E, int64_t())</code> is required to work, but <code>int64_t l = 0; views::drop(E, l)</code> 
is required to be ill-formed. This seems unlikely to be the intent either.
</p>

<p><i>[2021-03-12; Reflector poll]</i></p>

<p>
Set priority to 3 following reflector poll.
</p>

<p><i>[2021-06-13 Resolved by the adoption of <a href="https://wg21.link/P2367R0" title=" Remove misuses of list-initialization from Clause 24">P2367R0</a> at the June 2021 plenary. Status changed: New &rarr; Resolved.]</i></p>



<p id="res-3524"><b>Proposed resolution:</b></p>





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