Returning tuple of Green's functions does not compile if converted to python

[Stefan-Dienst]

Using TRIQS 2.2.1 with hash 3e0ed62 throws an error at compilation if I try to convert a function that returns a tuple of Green's functions to python.

I have setup an app4triqs skeleton with a minimal example: https://github.com/StefanKaeser/tests_for_triqs.
If I convert the function

std::tuple<g_wk_t, g_wk_t> test_tuple(g_wk_t g_wk) {
  
      return {g_wk, g_wk};
  }

in tests_for_triqs.cpp to python, using cpp2py, I can no longer compile.
Clang version 8.0.1 throws the following error:

In file included from /home/kaeser/my_triqs_installations/tests_for_triqs.build/python/tests_for_triqs/tests_for_triqs_module_wrap.cxx:28:
/home/kaeser/my_triqs_installations/triqs_2.2.x/include/cpp2py/converters/tuple.hpp:15:91: error: 
      conversion function from 'const __tuple_element_t<0UL,                                         
      tuple<triqs::gfs::gf<triqs::gfs::cartesian_product<triqs::gfs::imfreq,                         
      triqs::lattice::brillouin_zone>, triqs::gfs::matrix_valued>,    
      triqs::gfs::gf<triqs::gfs::cartesian_product<triqs::gfs::imfreq,            
      triqs::lattice::brillouin_zone>, triqs::gfs::matrix_valued> > >' (aka 'const
      triqs::gfs::gf<triqs::gfs::cartesian_product<triqs::gfs::imfreq, triqs::lattice::brillouin_zone>, triqs::gfs::matrix_valued>') to
'cpp2py::py_converter<triqs::gfs::gf_view<triqs::gfs::cartesian_product<triqs::gfs::imfreq,                    
      triqs::lattice::brillouin_zone>, triqs::gfs::matrix_valued> >::c_type' (aka                                    
      'gf_view<triqs::gfs::cartesian_product<triqs::gfs::imfreq,                                                     
      triqs::lattice::brillouin_zone>, triqs::gfs::matrix_valued>') invokes a deleted  
      function                                                                             
  ...std::array<pyref, sizeof...(Is)>{pyref(py_converter<Types>::c2py(std::get<Is>(t)))....

Can anyone reproduce this or knows what is going on here?

[Wentzell]

Hi Stefan,

Thank you for pointing out this problem! This is in fact a bug in C++2py

@parcollet
In the tuple converter file c2py will take any tuple by const &, c.f. cpp2py/converters/tuple.hpp:14

template <std::size_t... Is> static PyObject *c2py_impl(tuple_t const &t, std::index_sequence<Is...>) 

The example Stefan gave will thus trigger a conversion of a gf const & at cpp2py/converters/tuple.hpp:14
which will try to construct a gf_view from a gf const & which leads to the problem above.

Shouldn't the tuple converter provide a && overload of c2py

template <std::size_t... Is> static PyObject *c2py_impl(tuple_t &&t, std::index_sequence<Is...>)

This seems to solve the problem above.

[Wentzell]

This is already fixed on triqs/unstable. TODO Add test

[Wentzell]

Test was added in 5b0a100