Serializing std::variant causes stack overflow

[untrioctium]

This may be similar to #1261, but version 3.3.0, on my tested compilers, will stack overflow trying to serialize a variant instead of yelling at compile time that it doesn't know how to do it.

With this code:

#include "json.hpp"
#include <variant>
#include <iostream>

int main() {
	std::variant<int, float, std::string> t = 3.14159f;
	std::cout << nlohmann::json(t).dump() << std::endl;
}

MSVC 19.14.26430 will produce the cryptic and lengthy error C4717:

'nlohmann::detail::json_ref<nlohmann::basic_json<std::map,std::vector,std::basic_string<char,std::char_traits<char>,std::allocator<char> >,bool,__int64,unsigned __int64,double,std::allocator,nlohmann::adl_serializer> >::json_ref<nlohmann::basic_json<std::map,std::vector,std::basic_string<char,std::char_traits<char>,std::allocator<char> >,bool,__int64,unsigned __int64,double,std::allocator,nlohmann::adl_serializer> ><std::variant<int,float,std::basic_string<char,std::char_traits<char>,std::allocator<char> > > &>': recursive on all control paths, function will cause runtime stack overflow

And crash with a stack overflow when run. GCC 7.3.0 and Clang 6.0.0 will not produce a compile error, but will segfault. An earlier version of json I was working with, 3.1.2, instead complains that it's not sure how to serialize a std::variant on all previously tested compilers.

json.hpp:1545:5: error: static assertion failed: could not find to_json() method in T's namespace
     static_assert(sizeof(BasicJsonType) == 0,

I cannot even begin to claim to know what the resolution of this problem might even vaguely look like, but I hope this is helpful information. Hopefully I'm not just doing something horribly wrong.

[theodelrieu]

Well... oops.

I'll make a fix asap

[nlohmann]

@untrioctium Could you please check whether #1294 fixes this issue?

[untrioctium]

@nlohmann Yes, on all previously tested compilers with that test code, I now get "static assertion failed: could not find to_json() method in T's namespace" using the fixed version. Thanks for the quick resolution @theodelrieu

[nlohmann]

Thanks for the quick check, @untrioctium ! And of course, @theodelrieu, thanks for the quick fix!

[zhongcy]

@nlohmann
Hi, variant is not support in Release 3.6.1? Have any Variant examples?
I test the above example is not success in Release 3.6.1.