Namespacing with external C++ and `--standalone-functions` causes issues

[andrjohns]

When calling --standalone-functions with undefined/external-c++, the generated functions expect the function definitions to be within the model namespace.

For example, with the make_odds example, the generated .hpp is:

// Code generated by stanc v2.35.0-rc1
#include <stan/model/model_header.hpp>
namespace external_model_namespace {
using stan::model::model_base_crtp;
using namespace stan::math;
stan::math::profile_map profiles__;
static constexpr std::array<const char*, 9> locations_array__ =
  {" (found before start of program)",
  " (in 'examples/external/external.stan', line 9, column 2 to column 31)",
  " (in 'examples/external/external.stan', line 16, column 2 to column 12)",
  " (in 'examples/external/external.stan', line 17, column 2 to column 26)",
  " (in 'examples/external/external.stan', line 12, column 2 to column 21)",
  " (in 'examples/external/external.stan', line 13, column 2 to column 23)",
  " (in 'examples/external/external.stan', line 5, column 2 to column 17)",
  " (in 'examples/external/external.stan', line 6, column 8 to column 9)",
  " (in 'examples/external/external.stan', line 6, column 2 to column 35)"};
}

// [[stan::function]]
double make_odds(const double& theta, std::ostream* pstream__ = nullptr) {
  return external_model_namespace::make_odds(theta, pstream__);
}

Compiling the generated hpp and including the user header fails, as the make_odds function wasn't declared in a namespace in the external user header.

This is a bit of an edge-case, so feel free to let me know if it would be more effort than it's worth to fix, and I can just workaround it in cmdstanr

[WardBrian]

Hm, this is an interesting edge case. It does seem like if you’re doing this, you’re doing something pretty weird, because you could just call your own header. If I was going to “fix” this I think they way I would want to do it is actually by just not emitting a definition at all in this case

[andrjohns]

An 'easy' fix could be to just emit the declaration for any undefined functions when standalone-functions is called. So the make_odds generation could just be:

// [[stan::function]]
double make_odds(const double& theta, std::ostream* pstream__);

Would that work?

[WardBrian]

That would break the compile of someone who needed their own forward decl, I think. For example, if make odds was recursive

It also would probably be an issue that the user header is included before the stanc generated code

[andrjohns]

Ah true. Yeah I'll just handle this internally in cmdstanr then