ADL mechanism seems to be changed in recent versions

[klezki]

Hi!

https://godbolt.org/z/T178Gxcq7
The code snippet above does not compile when using 3.11.1, and this is what I would expect I guess, but if you switch to "trunk" version (which is most likely 3.12 currently) it compiles successfully with both gcc and clang.

It seems that somewhere between 3.11.1 and 3.12 there was some change so now the compiler is somehow able to find default nlohmann::to_json overload via ADL.

I've tried to look into it myself but I couldn't find any changes that may caused this, and tbh I don't really understand how come it works in the first place currently. Because as far as I can see we shouldn't be able to find to_json overloads, since they are in details namespace, we only have to_json inline variable in nlohmann namespace, and it shouldnt be findable by adl.

So I would really appreciate if someone could explain how does it work and if its the intended behavior.

Happy new year btw!

[gregmarr]

CE shows that It's calling this function:

  nlohmann::detail::to_json<nlohmann::basic_json<>, char [7], 0>(nlohmann::basic_json<>&, char const (&) [7])

which is found here:
https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/conversions/to_json.hpp#L400-L408

I'm not sure exactly why it's finding that in the detail namespace given the types involved. Perhaps because it's a template function and the BasicJsonType is used there.

[klezki]

I think I found a reason
Its because basic_json now derives from nlohmann::detail::json_base_class, introduced here
Base class is one of the "associated entities" for ADL, and it includes its innermost namespace other than the lookup set, so that's why nlohmann::details is exposed to ADL now.

I doubt this side-effect was intended by the change above, so I guess it would be a bad idea to rely on it. Although there are already couple of places in our project (that I know of) that work only because of this :))).

[gregmarr]

Probably a "fortunate accident" that the to_json and from_json functions now work cleanly. It's good to know, however, that changing the base class could make those no longer found.

Update: no, the specified base class is wrapped by the detail type, so that will always be there.