unused parameter warnings in operator<< in boost::optional with clang++

[Moritz Bunkus <moritz@…>]

Trivial example program:

#include <boost/optional.hpp>

int
main(int, char *[]) {
  boost::optional<int> i;
}

Compile with clang++:

clang++ -Wall -Wextra -Wno-unused-local-typedefs \
  -I$HOME/opt/boost/boost_1_58_0/include/ \
  -L$HOME/opt/boost/boost_1_58_0/lib/ -o cpp2 cpp2.cpp

Output:

In file included from cpp2.cpp:1:
In file included from /home/mosu/opt/boost/boost_1_58_0/include/boost/optional.hpp:15:
/home/mosu/opt/boost/boost_1_58_0/include/boost/optional/optional.hpp:1254:53: warning: unused parameter 'out' [-Wunused-parameter]
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional_detail::optional_tag const& v)
                                                    ^
/home/mosu/opt/boost/boost_1_58_0/include/boost/optional/optional.hpp:1254:95: warning: unused parameter 'v' [-Wunused-parameter]
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional_detail::optional_tag const& v)
                                                                                              ^
2 warnings generated.

This is a regression from 1.57.0 where no such warnings were emitted.

clang++ 3.6.0 emits warnings with 1.58.0 but not with 1.57.0.

g++ 4.9.2 emits warnings with neither of those versions.