rvalue stream operators (1203 and 2534)

[zoecarver]

Summary

This patch fixes issues 1203 and 2534. 1203 hasn't been adopted by the standard but was already partially supported. I think it is better to fully support it (which cannot break existing code) rather than remove partial support of it which will remove functionality people may rely on.

[zoecarver]

• update status

test/std/input.output/iostream.format/input.streams/istream.rvalue/perfect_forwarding.pass.cpp

1 ↗

(On Diff #203068)

I didn't include a test like this for ostream because I didn't think it would be meaningful (I can if you want though).

[zoecarver]

This may have slipped through the cracks — ping (but no rush).

[mclow.lists]

test/std/input.output/iostream.format/input.streams/istream.rvalue/ios_base_operator_overload_resolution.fail.cpp

31 ↗

(On Diff #203069)

Instead of std::move(is), you could just use std::istringstream()

test/std/input.output/iostream.format/input.streams/istream.rvalue/perfect_forwarding.pass.cpp

41 ↗

(On Diff #203069)

Why A and B here? What's the point of having two different types?
And do A and B have stream insertion operations?

[zoecarver]

• fix based on review

[zoecarver]

Does this look good? I would like to get it committed :)

[zoecarver]

Another ping (@mclow.lists)

[zoecarver]

Resolved by c90dee1 and D96885.

libcxx/include/istream

 #endif  // _LIBCPP_NO_EXCEPTIONS
         __is.setstate(__state);
     }
     return __is;
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
-template <class _CharT, class _Traits, class _Tp>
+template <class _Stream, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY
-basic_istream<_CharT, _Traits>&
-operator>>(basic_istream<_CharT, _Traits>&& __is, _Tp&& __x)
+typename enable_if
+<
+    (__has_in_stream_op<_Stream&, _Tp& >::value ||
+     __has_in_stream_op<_Stream&, _Tp&&>::value) &&
+    is_base_of<ios_base, _Stream>::value,
+    _Stream&&
+>::type
+operator>>(_Stream&& __is, _Tp&& __x)
 {
     __is >> _VSTD::forward<_Tp>(__x);
-    return __is;
+    return _VSTD::move(__is);
 }
 
 #endif  // _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS basic_iostream
     : public basic_istream<_CharT, _Traits>,
       public basic_ostream<_CharT, _Traits>

libcxx/include/ostream

 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _Stream, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY
 typename enable_if
 <
-    !is_lvalue_reference<_Stream>::value &&
+    (__has_out_stream_op<_Stream&,  _Tp&>::value ||
+     __has_out_stream_op<_Stream&&, _Tp&>::value) &&
     is_base_of<ios_base, _Stream>::value,
     _Stream&&
 >::type
 operator<<(_Stream&& __os, const _Tp& __x)
 {
     __os << __x;
     return _VSTD::move(__os);
 }

libcxx/include/type_traits

 #else
 inline _LIBCPP_CONSTEXPR
 bool __libcpp_is_constant_evaluated() _NOEXCEPT { return false; }
 #endif
 
 template <class _CharT>
 using _IsCharLikeType = _And<is_standard_layout<_CharT>, is_trivial<_CharT> >;
 
+#ifndef _LIBCPP_CXX03_LANG
+// __has_stream_op
+
+template<class _Tp, class _Up>
+struct __has_out_stream_op
+{
+  template<class _T1, class _U1>
+  static auto test(int) -> decltype(declval<_T1>() << declval<_U1>());
+
+  template<class, class>
+  static auto test(...) -> false_type;
+
+  static constexpr bool value = !is_same<false_type, decltype(test<_Tp, _Up>(0))>::value;
+};
+
+template<class _Tp, class _Up>
+struct __has_in_stream_op
+{
+  template<class _T1, class _U1>
+  static auto test(int) -> decltype(declval<_T1>() >> declval<_U1>());
+
+  template<class, class>
+  static auto test(...) -> false_type;
+
+  static constexpr bool value = !is_same<false_type, decltype(test<_Tp, _Up>(0))>::value;
+};
+
+#endif  // _LIBCPP_CXX03_LANG
+
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
 // std::byte
 namespace std  // purposefully not versioned
 {
 template <class _Integer>
   constexpr typename enable_if<is_integral_v<_Integer>, byte>::type &

libcxx/test/std/input.output/iostream.format/input.streams/istream.rvalue/ios_base_operator_overload_resolution.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <istream>
+
+// template <class charT, class traits = char_traits<charT> >
+//   class basic_istream;
+
+// template <class charT, class traits, class T>
+//   basic_ostream<charT, traits>&
+//   operator>>(basic_istream<charT, traits>&& os, const T& x);
+
+// LWG: 2534 and 1203
+
+#include <sstream>
+
+#include "test_macros.h"
+
+struct A {};
+
+int main(int, char**)
+{
+    std::istringstream() >> A{}; // expected-error-re {{invalid operands to binary expression {{.*}}}}
+
+    return 0;
+}

libcxx/test/std/input.output/iostream.format/input.streams/istream.rvalue/perfect_forwarding.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <istream>
+
+// template <class charT, class traits, class T>
+//   basic_istream<charT, traits>&
+//   operator>>(basic_istream<charT, traits>&& is, T&& x);
+
+#include <istream>
+#include <sstream>
+#include <cassert>
+
+#include "test_macros.h"
+
+struct Foo{};
+
+bool rval = false;
+bool lval = false;
+
+void operator>>(std::istream&, Foo&&){ rval = true; }
+void operator>>(std::istream&, Foo& ){ lval = true; }
+
+int main(int, char**)
+{
+    {
+        std::istringstream() >> Foo{};
+        assert(rval);
+    }
+    {
+        Foo foo;
+        std::istringstream() >> foo;
+        assert(lval);
+    }
+
+    return 0;
+}

libcxx/www/cxx1z_status.html

<!--   <I>Note: "NAD" means that the issue was deemed "Not a defect"</I> -->
   <tr><td><a href="https://wg21.link/LWG2510">2510</a></td><td>Tag types should not be DefaultConstructible</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2514">2514</a></td><td>Type traits must not be final</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2518">2518</a></td><td>[fund.ts.v2] Non-member swap for propagate_const should call member swap</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2519">2519</a></td><td>Iterator operator-= has gratuitous undefined behaviour</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2521">2521</a></td><td>[fund.ts.v2] weak_ptr's converting move constructor should be modified as well for array support</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2525">2525</a></td><td>[fund.ts.v2] get_memory_resource should be const and noexcept</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2527">2527</a></td><td>[fund.ts.v2] ALLOCATOR_OF for function::operator= has incorrect default</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2531">2531</a></td><td>future::get should explicitly state that the shared state is released</td><td>Issaquah</td><td></td></tr>
-  <tr><td><a href="https://wg21.link/LWG2534">2534</a></td><td>Constrain rvalue stream operators</td><td>Issaquah</td><td></td></tr>
+  <tr><td><a href="https://wg21.link/LWG2534">2534</a></td><td>Constrain rvalue stream operators</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2536">2536</a></td><td>What should &lt;complex.h&gt; do?</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2540">2540</a></td><td>unordered_multimap::insert hint iterator</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2543">2543</a></td><td>LWG 2148 (hash support for enum types) seems under-specified</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2544">2544</a></td><td>istreambuf_iterator(basic_streambuf<charT, traits>* s) effects unclear when s is 0</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2556">2556</a></td><td>Wide contract for future::share()</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2562">2562</a></td><td>Consistent total ordering of pointers by comparison functors</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2567">2567</a></td><td>Specification of logical operator traits uses BaseCharacteristic, which is defined only for UnaryTypeTraits and BinaryTypeTraits</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2568">2568</a></td><td>[fund.ts.v2] Specification of logical operator traits uses BaseCharacteristic, which is defined only for UnaryTypeTraits and BinaryTypeTraits</td><td>Issaquah</td><td></td></tr>