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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [regression] std::array<int, 0>::begin() should be constexpr but isn't"
href="https://bugs.llvm.org/show_bug.cgi?id=40124">40124</a>
</td>
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<th>Summary</th>
<td>[regression] std::array<int, 0>::begin() should be constexpr but isn't
</td>
</tr>
<tr>
<th>Product</th>
<td>libc++
</td>
</tr>
<tr>
<th>Version</th>
<td>7.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>All Bugs
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<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>tonyelewis@hotmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
</tr></table>
<p>
<div>
<pre>Compiling the following with `-fsyntax-only -std=c++17 -stdlib=libc++` :
~~~
#include <array>
template <typename Range>
inline constexpr bool f(const Range &prmRange) {
prmRange.begin();
return true;
}
void some_function() {
static_assert( f( ::std::array<int, 1>{ 0 } ) );
static_assert( f( ::std::array<int, 0>{ } ) );
}
~~~
I get:
~~~
a.cpp:11:17: error: static_assert expression is not an integral constant
expression
static_assert( f( ::std::array<int, 0>{ } ) );
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cpp:5:11: note: non-constexpr function 'begin' cannot be used in a constant
expression
prmRange.begin();
^
a.cpp:11:17: note: in call to 'f(::std::array<int, 0>{})'
static_assert( f( ::std::array<int, 0>{ } ) );
^
/home/lewis/source/llvm/bin/../include/c++/v1/array:273:20: note: declared here
const_iterator begin() const _NOEXCEPT {return const_iterator(data());}
^
1 error generated.
~~~
It looks like the zero-size specialisation of `::std::array` has some
constexprs missing. From what I can see in "26.3.7.1 Class template array
overview [array.overview]" and "26.3.7.8 Zero sized arrays [array.zero]" of the
C++17 draft, a zero-sized array should have constexpr `begin()` (etc).
This appears to be a regression: on Godbolt, I'm seeing this fail to compile in
this way under 7.0 and trunk, but compile cleanly under 6.0.
Thanks very much for all work on libc++.</pre>
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