istream to bool conversion: when is it available?

Question

This simple code

bool foo(std::istringstream&stream, std::string&single, char del)
{ return std::getline(stream,single,del); }

compiles with gcc (4.8.2) but not with clang (3.4, using libc++), which complains that there is no viable conversion from std::basic_istream<char, std::char_traits<char> > to bool. However, when I wrap argument to the return statement in a static_cast<bool>(), clang is happy.

This confused me made me wonder whether the above code is well formed or not, i.e. whether gcc or clang is correct. According to cpprefernce std::getline returns a std::basic_istream<char, std::char_traits<char> >, which is inherited from a std::basic_ios which has the type conversion operator bool (since C++11, before it was a type conversion to void*). Shouldn't this conversion operator get selected automatically? (for some reason, I'm more ready to accept that gcc is wrong than clang).

Edit I just figured out that apparently libc++ of llvm declares the conversion operator in question explicit, deeming it invalid for the implicit conversion. Is this in line with the standard?

Answer 1

clang is correct. Since C++11, the conversion from std::basic_ios to bool is indeed required to be explicit.

C++11, [ios.overview]:

explicit operator bool() const;

Answer 2

As the initialization that occurs in function return is the copy-initialization then the explicit conversion operator bool may not be applied implicitly. From the C++ Standard

2 A conversion function may be explicit (7.1.2), in which case it is only considered as a user-defined conversion for direct-initialization (8.5).

So GCC has a bug.

The operator may be applied implicitly when the direct-initialization is used or in special context as the context of the if condition.