Variable is wrongfully captured by macro expansion

[lschuetze]

A macro expansion captures a variable wrongfully leading to code compilation errors.
The first expansion of CELL_MEMBAL(,) is okay, because shift is not defined, yet. The second expansion results in the error shown below.

The code is the generated code from the LJ_paper/Lennard Jones example.

  auto particles_cellList_0 = particles.getCellList<CELL_MEMBAL(3, double)>(r_cut);
...
  double shift = (2.0 * ((sigma12 / (((((r_cut2 * r_cut2) * r_cut2) * r_cut2) * r_cut2) * r_cut2)) - (sigma6 / ((r_cut2 * r_cut2) * r_cut2))));
...
  auto cellList = particles.getCellList<CELL_MEMBAL(3, double)>(r_cut);

In file included from main.cpp:2:
main.cpp: In function ‘int main(int, char**)’:
main.cpp:76:53: error: the value of ‘shift’ is not usable in a constant expression
   76 |   auto cellList = particles.getCellList<CELL_MEMBAL(3, double)>(r_cut);
      |                                                     ^
/net/home/lschuetze/openfpm/bin/openfpm_pdata/include/Vector/vector_dist.hpp:177:86: note: in definition of macro ‘CELL_MEMBAL’
  177 | CELL_MEMBAL(dim,St) CellList_gen<dim, St, Process_keys_lin, Mem_bal<>, shift<dim, St> >
      |                                                                              ^~~

main.cpp:73:10: note: ‘shift’ was not declared ‘constexpr’
   73 |   double shift = (2.0 * ((sigma12 / (((((r_cut2 * r_cut2) * r_cut2) * r_cut2) * r_cut2) * r_cut2)) - (sigma6 / ((r_cut2 * r_cut2) * r_cut2))));
      |          ^~~~~
In file included from main.cpp:2:
/net/home/lschuetze/openfpm/bin/openfpm_pdata/include/Vector/vector_dist.hpp:177:93: error: type/value mismatch at argument 5 in template parameter list for ‘template<unsigned int dim, class T, template<unsigned int <anonymous>, class> class Prock, class Mem_type, class transform, class vector_pos_type> class CellList_gen’
  177 | ,St) CellList_gen<dim, St, Process_keys_lin, Mem_bal<>, shift<dim, St> >
      |                                                                      ^

main.cpp:76:41: note: in expansion of macro ‘CELL_MEMBAL’
   76 |   auto cellList = particles.getCellList<CELL_MEMBAL(3, double)>(r_cut);
      |                                         ^~~~~~~~~~~
/net/home/lschuetze/openfpm/bin/openfpm_pdata/include/Vector/vector_dist.hpp:177:93: note:   expected a type, got ‘(shift < (double)3)’
  177 | ,St) CellList_gen<dim, St, Process_keys_lin, Mem_bal<>, shift<dim, St> >
      |                                                                      ^

main.cpp:76:41: note: in expansion of macro ‘CELL_MEMBAL’
   76 |   auto cellList = particles.getCellList<CELL_MEMBAL(3, double)>(r_cut);
      |                                         ^~~~~~~~~~~
main.cpp:76:63: error: invalid operands of types ‘<unresolved overloaded function type>’ and ‘double’ to binary ‘operator>’
   76 |   auto cellList = particles.getCellList<CELL_MEMBAL(3, double)>(r_cut);
      |                                                               ^~~~~~~~
      |                                                                 |
      |                                                                 double

[lschuetze]

The variable definition shift in line 73 masks the definition of the shift introduced by the macro CELL_MEMBRAL which is expanded to CellList_gen<dim, St, Process_keys_lin, Mem_bal<>, shift<dim, St> >.

We could change our code generator such that the definition of cell lists is always before user defined variables so there is no mismatch anymore. I applied this solution by hand and it is sufficient.