[Bug libstdc++/65861] New: libstdc++ is silently generating wrong code when its std::search is given an input iterator

["howarth.at.gcc at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65861

            Bug ID: 65861
           Summary: libstdc++ is silently generating wrong code when its
                    std::search is given an input iterator
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: howarth.at.gcc at gmail dot com

While porting rstudio 0.98.1103 to build under clang/libc++, I uncovered a
defect in the libstdc++ handling of input iterators by std::search
(https://llvm.org/bugs/show_bug.cgi?id=23307). The reduced test case is
silently generating wrong code when its std::search is given an input iterator
(it assumes it can save the iterator position and backtrack).

#include <string>
#include <iterator>
#include <algorithm>
void f(std::istream &s, std::string x) {
      std::istreambuf_iterator<char> eod;
      std::search(std::istreambuf_iterator<char>(s),
                  eod,
                  x.begin(),
                  x.end());
}

Clang with libc++ properly rejects this with the error...

In file included from test3.cpp:1:
In file included from
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:439:
In file included from
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:628:
In file included from
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:604:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/iterator:849:22:
error: 
      implicit instantiation of undefined template
'std::__1::basic_istream<char, std::__1::char_traits<char> >'
        : __sbuf_(__s.rdbuf()) {}
                     ^
test3.cpp:6:19: note: in instantiation of member function
'std::__1::istreambuf_iterator<char,
      std::__1::char_traits<char> >::istreambuf_iterator' requested here
      std::search(std::istreambuf_iterator<char>(s),
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/iosfwd:109:33:
note: 
      template is declared here
    class _LIBCPP_TYPE_VIS_ONLY basic_istream;
                                ^
In file included from test3.cpp:1:
In file included from
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:439:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:1572:12:
error: 
      no matching function for call to '__search'
    return _VSTD::__search<typename
add_lvalue_reference<_BinaryPredicate>::type>
          
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__config:358:15:
note: 
      expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE
              ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:1586:19:
note: 
      in instantiation of function template specialization
'std::__1::search<std::__1::istreambuf_iterator<char,
      std::__1::char_traits<char> >, std::__1::__wrap_iter<char *>,
std::__1::__equal_to<char, char> >' requested here
    return _VSTD::search(__first1, __last1, __first2, __last2, __equal_to<__v1,
__v2>());
                  ^
test3.cpp:6:12: note: in instantiation of function template specialization
      'std::__1::search<std::__1::istreambuf_iterator<char,
std::__1::char_traits<char> >, std::__1::__wrap_iter<char *>
      >' requested here
      std::search(std::istreambuf_iterator<char>(s),
           ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:1456:1:
note: 
      candidate function [with _BinaryPredicate = std::__1::__equal_to<char,
char> &, _RandomAccessIterator1 =
      std::__1::istreambuf_iterator<char, std::__1::char_traits<char> >,
_RandomAccessIterator2 =
      std::__1::__wrap_iter<char *>] not viable: no known conversion from
'typename
      std::iterator_traits<istreambuf_iterator<char, char_traits<char> >
>::iterator_category'
      (aka 'std::__1::input_iterator_tag') to
'std::__1::random_access_iterator_tag' for 6th argument
__search(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:1419:1:
note: 
      candidate function [with _BinaryPredicate = std::__1::__equal_to<char,
char> &, _ForwardIterator1 =
      std::__1::istreambuf_iterator<char, std::__1::char_traits<char> >,
_ForwardIterator2 = std::__1::__wrap_iter<char
      *>] not viable: no known conversion from 'typename
std::iterator_traits<istreambuf_iterator<char, char_traits<char>
      > >::iterator_category' (aka 'std::__1::input_iterator_tag') to
'std::__1::forward_iterator_tag' for 6th argument
__search(_ForwardIterator1 __first1, _ForwardIterator1 __last1,
^
2 errors generated.