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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::bit_cast falls over, seemingly due to some invisible alignment requirements"
href="https://bugs.llvm.org/show_bug.cgi?id=51925">bug 51925</a>
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<th>What</th>
<th>Removed</th>
<th>Added</th>
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<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>INVALID
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<td style="text-align:right;">Status</td>
<td>NEW
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<td>RESOLVED
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - std::bit_cast falls over, seemingly due to some invisible alignment requirements"
href="https://bugs.llvm.org/show_bug.cgi?id=51925#c1">Comment # 1</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - std::bit_cast falls over, seemingly due to some invisible alignment requirements"
href="https://bugs.llvm.org/show_bug.cgi?id=51925">bug 51925</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>This behavior is correct. Your type `Item` contains 9 bytes with determinate
values, followed by three padding bytes which have indeterminate values, but
your type `Data` contains a 12 byte array. The `bit_cast` therefore leaves the
last three bytes of that array with indeterminate values.
But because `bit_cast` returns by value, this means your program attempts to
copy those three `char` objects that have indeterminate values. Copying an
indeterminate value is only permitted for unsigned narrow character types and
`std::byte`, as the diagnostic says.
So this call to `bit_cast` results in undefined behavior, which means it's not
permitted in a constant expression.</pre>
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