Seeking breaks iostreams::restriction<std::istream> internal state

[chris42f@…]

As in the attached test case, calling seekg() on a restricted istream appears to break the internal state such that more characters can be extracted than expected.

I've done some digging around, and I think this is because the restriction device does a seek relative to std::ios_base::cur, which returns -1 from the underlying implementation: Calling std::streambuf::pubseekoff() with way = ios_base::cur and which = (ios_base::in | ios_base::out) fails according to the C++ standard section 27.8.2.4.

When fixing this, it's worth noting that calling pubseekoff() will also fail with libstdc++ if it's passed (ios_base::in | ios_base::out) when the stream buffer has only an output buffer or input buffer, but not both.

Tested with g++-4.6.3 and g++-4.8.? on 64 bit linux Ubuntu. Behaviour is consistent with clang++-libc++, but inconsistent with MSVC.

[chris42f@…]

Test case showing the bad behaviour