Bad seek behavior with libc++ (svn, rev 192988). There's a patch down there.

bad-seek.cpp

#include <cassert>
#include <streambuf>
#include <iostream>

// Dummy stream buffer for testing.
// basic_streambuf defines seekoff() and seekpos() (virtuals) to
// return pos_type(off_type(-1)), which indicates a seek failure, so
// we don't really have to define them, but I've done so for the sake
// of pedantry.

class dummy_streambuf
	: public std::basic_streambuf<char>
{
private:
	using base = std::basic_streambuf<char>;
	using base::pos_type;
	using base::off_type;

	char m_sentinel;

public:
	dummy_streambuf()
		: base()
		, m_sentinel(0x2A)
	{
		this->setp(&m_sentinel, &m_sentinel + 1u);
		this->setg(&m_sentinel, &m_sentinel, &m_sentinel + 1u);
	}

private:
	pos_type
	seekoff(
		off_type /*off*/,
		std::ios_base::seekdir /*way*/,
		std::ios_base::openmode /*which*/
	) override {
		return pos_type{off_type{-1}};
	}
 
	pos_type
	seekpos(
		pos_type /*pos*/,
		std::ios_base::openmode /*which*/
	) override {
		return pos_type{off_type{-1}};
	}
};

void
testbits(
	std::iostream const& stream
) {
	using ios = std::ios_base;
	ios::iostate const states = stream.rdstate();

	std::cout
		<< "states: " << states << ' '
		<< (states & ios::badbit  ? " " : "!") << "b "
		<< (states & ios::failbit ? " " : "!") << "f "
		<< (states & ios::eofbit  ? " " : "!") << "e "
		<< '\n'
	;
}

signed
main() {
	dummy_streambuf sb{};
	std::iostream stream{&sb};

	// All asserts here fail on libc++ rev 192988, but pass on
	// libstdc++ 4.7.3. The standard mandates that seekp() and seekg()
	// should set failbit if pubseekoff() or pubseekpos() fail.

	// libc++'s implementations of seekp(off, which) and
	// seekg(off, which) ignore the return value of pubseekoff()
	// (see patch).

	// seek beg: -1 (before beginning)
	stream.clear();
	stream.seekp(-1, std::ios_base::beg);
	testbits(stream); assert(!stream.good());

	stream.clear();
	stream.seekg(-1, std::ios_base::beg);
	testbits(stream); assert(!stream.good());

	// seek cur: 0 (mandated failure because of dummy_streambuf
	// implementation -- or rather: the lack thereof)
	stream.clear();
	stream.seekp( 0, std::ios_base::cur);
	testbits(stream); assert(!stream.good());

	stream.clear();
	stream.seekg( 0, std::ios_base::cur);
	testbits(stream); assert(!stream.good());

	// seek end: +1 (past end)
	stream.clear();
	stream.seekp(+1, std::ios_base::end);
	testbits(stream); assert(!stream.good());

	stream.clear();
	stream.seekg(+1, std::ios_base::end);
	testbits(stream); assert(!stream.good());

	return 0;
}

bad-seek.patch

Index: include/istream
===================================================================
--- include/istream	(revision 192988)
+++ include/istream	(working copy)
@@ -1369,8 +1369,10 @@
         this->clear(this->rdstate() & ~ios_base::eofbit);
         sentry __sen(*this, true);
         if (__sen)
+        {
             if (this->rdbuf()->pubseekpos(__pos, ios_base::in) == pos_type(-1))
                 this->setstate(ios_base::failbit);
+        }
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
@@ -1391,7 +1393,10 @@
 #endif  // _LIBCPP_NO_EXCEPTIONS
         sentry __sen(*this, true);
         if (__sen)
-            this->rdbuf()->pubseekoff(__off, __dir, ios_base::in);
+        {
+            if (this->rdbuf()->pubseekoff(__off, __dir, ios_base::in) == pos_type(-1))
+                this->setstate(ios_base::failbit);
+        }
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
Index: include/ostream
===================================================================
--- include/ostream	(revision 192988)
+++ include/ostream	(working copy)
@@ -1173,7 +1173,10 @@
 basic_ostream<_CharT, _Traits>::seekp(off_type __off, ios_base::seekdir __dir)
 {
     if (!this->fail())
-        this->rdbuf()->pubseekoff(__off, __dir, ios_base::out);
+    {
+        if (this->rdbuf()->pubseekoff(__off, __dir, ios_base::out) == pos_type(-1))
+            this->setstate(ios_base::failbit);
+    }
     return *this;
 }
 

Apparently my skimming of the standard was insufficient (I blame the iostream library). The standard mis-specifies seekp() and seekg(). LWG issue is forthcoming (thanks to marshall on irc.oftc.net/#llvm).

The LWG issue is #2341