Fix gcc 7 warning

[jagerman]

Under gcc 7 with -std=c++11, compilation results in several of the following warnings:

    In file included from /home/jagerman/src/pybind11/tests/test_sequences_and_iterators.cpp:13:0:
    /home/jagerman/src/pybind11/include/pybind11/operators.h: In function ‘pybind11::detail::op_<(pybind11::detail::op_id)0, (pybind11::detail::op_type)0, pybind11::detail::self_t, pybind11::detail::self_t> pybind11::detail::operator+(const pybind11::detail::self_t&, const pybind11::detail::self_t&)’:
    /home/jagerman/src/pybind11/include/pybind11/operators.h:78:76: warning: inline declaration of ‘pybind11::detail::op_<(pybind11::detail::op_id)0, (pybind11::detail::op_type)0, pybind11::detail::self_t, pybind11::detail::self_t> pybind11::detail::operator+(const pybind11::detail::self_t&, const pybind11::detail::self_t&)’ follows declaration with attribute noinline [-Wattributes]
     inline op_<op_##id, op_l, self_t, self_t> op(const self_t &, const self_t &) {         \
                                                                                ^
    /home/jagerman/src/pybind11/include/pybind11/operators.h:109:1: note: in expansion of macro ‘PYBIND11_BINARY_OPERATOR’
     PYBIND11_BINARY_OPERATOR(add,       radd,         operator+,    l + r)
     ^~~~~~~~~~~~~~~~~~~~~~~~
    In file included from /home/jagerman/src/pybind11/include/pybind11/cast.h:15:0,
                     from /home/jagerman/src/pybind11/include/pybind11/attr.h:13,
                     from /home/jagerman/src/pybind11/include/pybind11/pybind11.h:36,
                     from /home/jagerman/src/pybind11/tests/pybind11_tests.h:2,
                     from /home/jagerman/src/pybind11/tests/test_sequences_and_iterators.cpp:11:
    /home/jagerman/src/pybind11/include/pybind11/descr.h:116:36: note: previous definition of ‘pybind11::detail::descr pybind11::detail::operator+(pybind11::detail::descr&&, pybind11::detail::descr&&)’ was here
         PYBIND11_NOINLINE descr friend operator+(descr &&d1, descr &&d2) {
                                        ^~~~~~~~

This appears to be happening because gcc is considering implicit construction of descr in some places using addition of two descr-compatible arguments in the descr.h c++11 fallback code. There's no particular reason that this operator needs to be a friend function: this commit changes it to an rvalue-context member function operator, which avoids the warning.

[jagerman]

This seems a pretty straightforward change. Does anyone see a potential issue I've overlooking with the friend function -> member operator change?

[dean0x7d]

Why is this even a warning? GCC 7 bug? Those are completely unrelated declarations.

[jagerman]

I know, I found it rather odd as well, and it definitely smells like a bug—but this is in the final GCC 7.1 stable release, so the workaround seems warranted. The patch also doesn't actually even "work around" anything, but rather just slightly tightens up the contexts where descr addition can be used, which seems worthwhile anyway (though below the level at which I'd bother fixing were it not for the warning).

This same warning may be triggered in other places as well, but we disable -Wattributes in pybind11.h. operators.h lives outside that, though, which is why it gets exposed.

[wjakob]

LGTM.