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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - libc++ incorrectly treats operator>>(istream& unsigned int&) as an error when converting negative values"
href="https://bugs.llvm.org/show_bug.cgi?id=36914">36914</a>
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<th>Summary</th>
<td>libc++ incorrectly treats operator>>(istream& unsigned int&) as an error when converting negative values
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>6.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>billy.oneal@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
<div>
<pre>Consider the following program:
#include <iostream>
#include <sstream>
int main()
{
std::istringstream a("-1");
unsigned i;
a >> i;
if (a.fail())
std::cout << "Bug\n";
else
std::cout << "Ok\n";
std::istringstream b("4294967296");
b >> i;
if (b.fail())
std::cout << "Ok\n";
else
std::cout << "Bug\n";
return 0;
}
libc++ thinks that -1 isn't representable in the type unsigned int, and so
treats this as an error. However, the standard disagrees with this. N4727
[istream.formatted.arithmetic] says that the operator>> calls locale's num_get.
[facet.num.get.virtuals]/3 says it reads as if by scanf %u. C11 7.21.6.2/12
says that %u behaves as if by strtoul. C11 7.22.1.4/5 says:
If the subject sequence begins with a minus sign, the value resulting from
the conversion is negated (in the return type).
That means that the parsing should remember that there was a minus sign, parse
the number as an unsigned int (with usual ERANGE checks), then, because the
input started with a minus sign, negate the resulting value.
(I ran into this bug when a user reported that they expected MSVC++ to treat -1
input as an error here; but libstdc++ agrees with us in not setting failbit
here, and C's rules, however bizarre, do not support setting failbit here)</pre>
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