istreambuf_iterator constructed with istream at eof isn't detected

[cjdb]

When comparing a freshly-constructed istreambuf_iterator with an istream that has already reached EOF, istreambuf_iterator != default_sentinel returns true. This behaviour differs from std::istreambuf_iterator.

#include <experimental/ranges/iterator>
#include <iostream>
#include <sstream>

int main()
{
   auto in = std::istringstream{"hello"};
   for (auto i = __stl2::istreambuf_iterator<char>{in}; i != __stl2::default_sentinel{}; ++i) {
      std::cout << *i << '\n';
   }
   for (auto i = __stl2::istreambuf_iterator<char>{in}; i != __stl2::default_sentinel{}; ++i) {
      std::cout << *i << '\n'; // Line 12
   }
}

Interestingly, this is only on the first pass: commenting out line 12 causes the program to complete without error.

[cjdb]

This might be related to #28.

[CaseyCarter]

istreambuf_iterator is, unsuprisingly, broken. [istreambuf.iterator.cons]:

1 For each istreambuf_­iterator constructor in this subclause, an end-of-stream iterator is constructed if and only if the exposition-only member sbuf_­ is initialized with a null pointer value.

constexpr istreambuf_iterator() noexcept;

2 Effects: Initializes sbuf_­ with nullptr.

istreambuf_iterator(istream_type& s) noexcept;

3 Effects: Initializes sbuf_­ with s.rdbuf().

istreambuf_iterator(streambuf_type* s) noexcept;

4 Effects: Initializes sbuf_­ with s.

istreambuf_iterator(const proxy& p) noexcept;

5 Effects: Initializes sbuf_­ with p.sbuf_­.

[istreambuf.iterator.ops] para 5 & 6:

bool equal(const istreambuf_iterator& b) const;

5 Returns: true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use.

template<class charT, class traits>
  bool operator==(const istreambuf_iterator<charT,traits>& a,
                  const istreambuf_iterator<charT,traits>& b);

6 Returns: a.equal(b)

Yet this program:

#include <cassert>
#include <iterator>
#include <sstream>

int main()
{
   auto in = std::istringstream{};
   assert(in.rdbuf() != nullptr);
   using I = std::istreambuf_iterator<char>;
   // Since in.rdbuf() != nullptr, I{in} does not construct an end-of-stream iterator. Right?
   assert(I{in} != I{}); 
}

Fails on on libstdc++/libc++/STLSTL.