`PODIO_JSON_OUTPUT` not set downstream, but json support still compiled into libraries about podio

[tmadlener]

I can also reproduce this in my development environment. Quickest workaround for now seems to be to add a #define PODIO_JSON_OUTPUT to each macro that wants to use this. Obviously that is not a longterm solution.

(Adding a few thoughts that we already partially discussed in private):
I gave putting all to_json function implementations into the respective headers a quick go, and that doesn't really work because then we would also have to start replacing forward declarations with #includes of the corresponding headers, which most likely is not viable because relations can lead to cyclic includes here.

I am also not sure whether we can make, e.g. a #define EDM4HEP_PODIO_OUTPUT that we implicitly generate based on the value of PODIO_JSON_OUTPUT work nicely. Maybe the cleanest solution would be to make this explicitly opt-in by creating a new option in the YAML file that enables / disables the json conversion.

[wdconinc]

Another thought. Maybe we can make the current approach work? I don't actually understand what goes wrong in cling. Clearly it finds the dictionary and the (correct, for my system) nlohmann-json headers. So I don't really understand why it thinks the constructor signature doesn't match? Is this maybe due to an implicit c++ option that affects initializer lists?

[tmadlener]

Looking at the error message again, I think the problem is actually in an inline definition of a to_json function, as it is in TrackState.h which is a component. Not sure yet what that teaches us about the problem.

[tmadlener]

Maybe it would be easiest to remove the #ifdef PODIO_JSON_OUTPUT with a

#if __has_include("nlohmann/json.hpp")
  #include "nlohmann/json.hpp"
  // potentially a version check
  #define PODIO_JSON_OUTPUT // or something else for easier internal use
#endif

in that way we would at least not depend on some external -DPODIO_JSON_OUTPUT, but would have some (maybe) consistent internal check.

[wdconinc]

I think it seems like the Components just aren't guarded. Nevermind, that didn't make sense.