[libc++] Implement LWG 3199

[ldionne]

Summary

The resolution of LWG 3199 makes sure that input-streaming into an empty bitset
does not set the failbit on the input stream.

[ldionne]

Ping!

[mclow.lists]

LGTM.

libcxx/include/istream

#endif
                 _CharT __ch = _Traits::to_char_type(__i);
                 if (!_Traits::eq(__ch, __zero) && !_Traits::eq(__ch, __one))
                     break;
                 __str.push_back(__ch);
                 ++__c;
                  __is.rdbuf()->sbumpc();
             }
             __x = bitset<_Size>(__str);
-            if (__c == 0)
+            if (_Size > 0 && __c == 0)
                __state |= ios_base::failbit;
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
             __state |= ios_base::badbit;
             __is.__setstate_nothrow(__state);
             if (__is.exceptions() & ios_base::badbit)

libcxx/test/std/utilities/template.bitset/bitset.operators/stream_in.pass.cpp

 int main(int, char**)
 {
     {
         std::istringstream in("01011010");
         std::bitset<8> b;
         in >> b;
         assert(b.to_ulong() == 0x5A);
     }
+    {
+        // Make sure that input-streaming an empty bitset does not cause the
+        // failbit to be set (LWG 3199).
+        std::istringstream in("01011010");
+        std::bitset<0> b;
+        in >> b;
+        assert(b.to_string() == "");
+        assert(!in.bad());
+        assert(!in.fail());
+        assert(!in.eof());
+        assert(in.good());
+    }
 #ifndef TEST_HAS_NO_EXCEPTIONS
     {
         std::stringbuf sb;
         std::istream is(&sb);
         is.exceptions(std::ios::failbit);
 
         bool threw = false;
         try {

libcxx/www/upcoming_meeting.html

 <tr><td><a href="https://wg21.link/LWG3183">3183</a></td><td>Yes</td><td>Normative permission to specialize Ranges variable templates</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3184">3184</a></td><td>Yes</td><td>Inconsistencies in <tt>bind_front</tt> wording</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3185">3185</a></td><td>Yes</td><td>Uses-allocator construction functions missing <tt>constexpr</tt> and <tt>noexcept</tt></td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3186">3186</a></td><td>Yes</td><td><tt>ranges</tt> removal, partition, and <tt>partial_sort_copy</tt> algorithms discard useful information</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3187">3187</a></td><td>Yes</td><td><a href="https://wg21.link/p0591r4">P0591R4</a> reverted DR 2586 fixes to <tt>scoped_allocator_adaptor::construct()</tt></td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3191">3191</a></td><td>Yes</td><td><tt>std::ranges::shuffle</tt> synopsis does not match algorithm definition</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3196">3196</a></td><td>Yes</td><td><tt>std::optional&lt;T&gt;</tt> is ill-formed is <tt>T</tt> is an array</td><td>Cologne</td><td>Complete</td></tr>
 <tr><td><a href="https://wg21.link/LWG3198">3198</a></td><td>Yes</td><td>Bad constraint on <tt>std::span::span()</tt></td><td>Cologne</td><td>Complete</td></tr>
-<tr><td><a href="https://wg21.link/LWG3199">3199</a></td><td>Yes</td><td><tt>istream &gt;&gt; bitset&lt;0&gt;</tt> fails</td><td>Cologne</td><td></td></tr>
+<tr><td><a href="https://wg21.link/LWG3199">3199</a></td><td>Yes</td><td><tt>istream &gt;&gt; bitset&lt;0&gt;</tt> fails</td><td>Cologne</td><td>Complete</td></tr>
 <tr><td><a href="https://wg21.link/LWG3202">3202</a></td><td>Yes</td><td>P0318R1 was supposed to be revised</td><td>Cologne</td><td>Complete</td></tr>
 <tr><td><a href="https://wg21.link/LWG3206">3206</a></td><td>Yes</td><td><tt>year_month_day</tt> conversion to <tt>sys_days</tt> uses not-existing member function</td><td>Cologne</td><td>Complete</td></tr>
 <tr><td><a href="https://wg21.link/LWG3208">3208</a></td><td>Yes</td><td><tt>Boolean</tt>'s expression requirements are ordered inconsistently</td><td>Cologne</td><td>Nothing to do</td></tr>
 <tr><td><a href="https://wg21.link/LWG3209">3209</a></td><td>Yes</td><td>Expression in <tt>year::ok()</tt> returns clause is ill-formed</td><td>Cologne</td><td>Complete</td></tr>
 </table>
 </table>
 
 <h3>Issues to "Review"</h3>

 <li>3183 - We don't do ranges yet</li>
 <li>3184 - We don't do bind_front yet</li>
 <li>3185 - Let's make sure that the patch up for review does this (D58879)</li>
 <li>3186 - We don't do ranges yet</li>
 <li>3187 - We need to make sure this is fixed when we land PMRs</li>
 <li>3191 - We don't do ranges yet</li>
 <li>3196 - We already do this</li>
 <li>3198 - We already do this</li>
-<li>3199 - Louis</li>
 <li>3202 - We already do this</li>
 <li>3206 - We already do this; added a couple of explicit tests</li>
 <li>3208 - Nothing to do; wording cleanup</li>
 <li>3209 - We already do the right thing</li>
 </ul>
 
 
 
 <p>Last Updated: 1-Jul-2019</p>
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