Modern compiler will issue a following error whenever '#include <string>'
is done:

/nix/store/yxpwamjdapjcp53mmsdh1j2c9bc26h4k-libc++-3.7.1/include/c++/v1/string:1938:44:
error: 'basic_string<_CharT, _Traits, _Allocator>' is missing exception specification 'noexcept(is_nothrow_copy_constructible<allocator_type>::value)'
basic_string<_CharT, _Traits, _Allocator>::basic_string(const allocator_type& __a)
                                           ^
/nix/store/yxpwamjdapjcp53mmsdh1j2c9bc26h4k-libc++-3.7.1/include/c++/v1/string:1326:40:
note: previous declaration is here
    _LIBCPP_INLINE_VISIBILITY explicit basic_string(const allocator_type& __a)
                                       ^
1 error generated.

This happens because modern clang is more strict about checking
exception specification for forward declaration and definition.

http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/string?r1=242056&r2=242623&diff_format=h

This relies on
e9d60c5 ('libc++3.7: fix to use with modern compilers')

libc++3.7: fix to use with clang 3.9