[PATCH] D99005: [clang] Implement P2266 Simpler implicit move

[Stephan Bergmann via Phabricator via cfe-commits]

sberg added a comment.

And, again for the record, with a build of LibreOffice with clang-cl (and `-Xclang -std=c++2b`) on Windows, at least against the C++ standard library from Visual Studio 2019 version 16.20.2, I ran into two issues in the standard library itself, when using `std::getline` and `std::istream::operator>>`:

  In file included from C:/lo-clang/core/setup_native/source/win32/customactions/reg_dlls/reg_dlls.cxx:12:
  C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\string(66,12): error: non-const lvalue reference to type 'basic_istream<...>' cannot bind to a temporary of type 'basic_istream<...>'
      return _Istr;
             ^~~~~
  C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\string(78,12): note: in instantiation of function template specialization 'std::getline<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>' requested here
      return getline(_STD move(_Istr), _Str, _Delim);
             ^
  C:/lo-clang/core/setup_native/source/win32/customactions/reg_dlls/reg_dlls.cxx(197,17): note: in instantiation of function template specialization 'std::getline<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>' requested here
      while (std::getline(ss, sToken, L'|'))
                  ^
  1 error generated.
  make[1]: *** [C:/lo-clang/core/solenv/gbuild/LinkTarget.mk:298: C:/lo-clang/core/workdir/CxxObject/setup_native/source/win32/customactions/reg_dlls/reg_dlls.o] Error 1
  
  c:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.29.30037/include/string

and

  [build CXX] l10ntools/source/merge.cxx
  In file included from C:/lo-clang/core/l10ntools/source/merge.cxx:21:
  In file included from C:/lo-clang/core/include\sal/log.hxx:20:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\sstream:11:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\istream:11:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\ostream:11:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\ios:11:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\xlocnum:16:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\streambuf:11:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\xiosbase:12:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\system_error:14:
  In file included from C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\stdexcept:12:
  C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\xstring(4971,12): error: non-const lvalue reference to type 'basic_istream<...>' cannot bind to a temporary of type 'basic_istream<...>'
      return _Istr;
             ^~~~~
  C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1429~1.300/Include\xstring(4977,29): note: in instantiation of function template specialization 'std::operator>><char, std::char_traits<char>, std::allocator<char>>' requested here
      return _STD move(_Istr) >> _Str;
                              ^
  C:/lo-clang/core/l10ntools/source/merge.cxx(125,18): note: in instantiation of function template specialization 'std::operator>><char, std::char_traits<char>, std::allocator<char>>' requested here
      aInputStream >> sPoFile;
                   ^
  1 error generated.
  make[1]: *** [C:/lo-clang/core/solenv/gbuild/LinkTarget.mk:301: C:/lo-clang/core/workdir/CxxObject/l10ntools/source/merge.o] Error 1

In both cases, the reason is that library happens to implement the overload taking `basic_istream&` by forwarding to the overload taking `basic_istream&&` (and not the other way around, as libc++ and libstdc++ happen to do), and the fix is to replace

  return _Istr;

in the overloads taking `basic_istream&& _Istr` with

  return static_cast<basic_istream<_Elem, _Traits>&>(_Istr);

(No idea if any MSVC standard library developers are around here, who might want to be CC'ed on this.)


Repository:
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  https://reviews.llvm.org/D99005/new/

https://reviews.llvm.org/D99005