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<body><span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - std::indirect_array::operator= gives wrong result."
href="https://bugs.llvm.org/show_bug.cgi?id=34515">bug 34515</a>
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<tr>
<th>What</th>
<th>Removed</th>
<th>Added</th>
</tr>
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<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
</td>
</tr>
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<td style="text-align:right;">CC</td>
<td>
</td>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
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<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>INVALID
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - std::indirect_array::operator= gives wrong result."
href="https://bugs.llvm.org/show_bug.cgi?id=34515#c2">Comment # 2</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - std::indirect_array::operator= gives wrong result."
href="https://bugs.llvm.org/show_bug.cgi?id=34515">bug 34515</a>
from <span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span></b>
<pre>OK, so first off, what you're *trying* to do probably results in UB.
[indirect.array.assign]p2 says: "If the indirect_array specifies an element in
the valarray<T> object to which it refers more than once, the behavior is
undefined."
It doesn't seem obvious to me what happens if that is true of the source and
destination indirect_arrays, but the same rule would seem reasonable. That
matches what libstdc++ does -- in its implementation, the relevant pointers are
marked as __restrict__, so aliasing would result in UB.
However, what you're *actually* doing is relying on a MSVC and libstdc++ bug.
Here's how indirect_array's assignment operator is *supposed* to be declared:
const indirect_array& operator=(const indirect_array&) const;
That's what libc++ does. Despite the UB I mentioned above, that assignment
(probably) results in UB, but in practice assigns left-to-right, thus assigning
input[1] (== 1) to input[1], then assigning input[2] (== 2) to input[3], then
assigning input[3] (now == 2) to input[4], then assigning input[4] (now == 2)
to input[6]. And the answer is {1, 2, 2, 2}.
The MSVC / libstdc++ bug is that they declare the assignment operator as
const indirect_array& operator=(const indirect_array&);
(Note the lack of 'const'.) Your code does not call that function. Instead, it
calls
const indirect_array& operator=(const valarray&) const;
... after converting result2 to a valarray<int> (which reads all the elements
at once).
So your code probably has UB, and in any case it's the other two
implementations that are wrong :)</pre>
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