overload_cast failure with recent Intel compilers

[robertodr]

Issue description

As the title says, I can't make overload_cast to work with recent Intel compilers:

1. icpc version 18.0.3 (gcc version 7.3.0 compatibility)
2. icpc version 18.0.1 (gcc version 6.4.0 compatibility)
3. icpc version 17.0.4 (gcc version 6.4.0 compatibility)
4. icpc version 17.0.1 (gcc version 6.3.0 compatibility)

Reproducible example code

As a MWE, I've modified the cmake_example to use overload_cast and the latest release of pybind11:

#include <pybind11/pybind11.h>

int add(int i, int j) {
    return i + j;
}

double add(double a, double b) {
    return a + b;
}

namespace py = pybind11;

PYBIND11_MODULE(cmake_example, m) {
    m.doc() = R"pbdoc(
        Pybind11 example plugin
        -----------------------

        .. currentmodule:: cmake_example

        .. autosummary::
           :toctree: _generate

           add
           subtract
    )pbdoc";

    m.def("add", py::overload_cast<int, int>(&add), R"pbdoc(
        Add two numbers

        Some other explanation about the add function.
    )pbdoc");

    m.def("add", py::overload_cast<double, double>(&add), R"pbdoc(
        Add two numbers

        Some other explanation about the add function.
    )pbdoc");

    m.def("subtract", [](int i, int j) { return i - j; }, R"pbdoc(
        Subtract two numbers

        Some other explanation about the subtract function.
    )pbdoc");

#ifdef VERSION_INFO
    m.attr("__version__") = VERSION_INFO;
#else
    m.attr("__version__") = "dev";
#endif
}

I configure with cmake -H. -Bbuild -DCMAKE_CXX_COMPILER=icpc -DCMAKE_C_COMPILER=icc -DCMAKE_CXX_STANDARD=14 and I invariably get:

Scanning dependencies of target cmake_example
[ 50%] Building CXX object CMakeFiles/cmake_example.dir/src/main.cpp.o
/cluster/home/roberto/Workspace/robertodr/cmake_example/pybind11/include/pybind11/detail/common.h(755): error: static assertion failed with "pybind11::overload_cast<...> requires compiling in C++14 mode"
      static_assert(detail::deferred_t<std::false_type, Args...>::value,
      ^
          detected during instantiation of class "pybind11::overload_cast<Args...> [with Args=<int, int>]" at line 27 of "/cluster/home/roberto/Workspace/robertodr/cmake_example/src/main.cpp"

This seems to be due to the fact that PYBIND11_CPP14 is never defined for the Intel compilers in detail/common.h I added the following lines to the CMakeLists.txt to remedy for that:

target_compile_definitions(cmake_example
  PRIVATE
    $<$<CXX_COMPILER_ID:Intel>:PYBIND11_CPP14>
  )

Now I get the following error for toolchains 1 and 2 above:

Scanning dependencies of target cmake_example
[ 50%] Building CXX object CMakeFiles/cmake_example.dir/src/main.cpp.o
/cluster/home/roberto/Workspace/robertodr/cmake_example/pybind11/include/pybind11/detail/descr.h(74): error: expression must have a constant value
      static constexpr auto digits = descr<sizeof...(Digits), 0>({ ('0' + Digits)..., '\0' }, { nullptr });

this error for toolchain 4:

In file included from /cluster/home/roberto/Workspace/robertodr/cmake_example/pybind11/include/pybind11/attr.h(13),
                 from /cluster/home/roberto/Workspace/robertodr/cmake_example/pybind11/include/pybind11/pybind11.h(43),
                 from /cluster/home/roberto/Workspace/robertodr/cmake_example/src/main.cpp(1):
/cluster/home/roberto/Workspace/robertodr/cmake_example/pybind11/include/pybind11/cast.h(838): error: alias template "pybind11::detail::type_caster_base<type>::cast_op_type" may not be used in the type-id of the alias-declaration
      template <typename T> using cast_op_type = cast_op_type<T>;

but no error for toolchain 3.

To summarize:

• The PYBIND11_CPP14 preprocessor variable should be defined for the Intel compilers.
• The 2018 releases of the Intel compilers I have access to do not work with C++14.

For the latter, I can't suggest a solution, but I'd be glad to test anything you might suggest.

[adegomme]

I recently had the same problem with the latest Intel 19 compiler, as PYBIND11_CPP14 is disabled for all of them.
Enabling the flag in common.h fixed my issue and made it possible to compile our code without issues.

I just tried with your modified cmake_example, and it seems to be working fine, I don't see a compilation issue. So Intel 19 might support more of C++14 and fix your problem (tried with pybind version e2b884c ) .

It would be nice if pybind11 could add the PYBIND11_CPP14 in this case.

Best regards