Error occurred when converting nlohmann::json to std::any

[kotori2]

Description

I would like to store objects in a std::map<std::string, std::any>. But converting nlohmann::json to std::any throws error type must be string, but is

I can only reproduce this issue on Windows. macOS seems working fine.

Reproduction steps

Minimal test case:

#include <any>
#include <nlohmann/json.hpp>

int main() {
    nlohmann::json j;
    std::any &&a = j;
    return 0;
}

Expected vs. actual results

Expected: Runs without error
Actual: Exception: Exception 0xe06d7363 encountered at address 0x7ffa855c466c

Minimal code example

No response

Error messages

<unknown> 0x00007ffa855c466c
<unknown> 0x00007ffa6dcfb660
nlohmann::detail::from_json<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::vector<unsigned char,std::allocator<unsigned char> > >,std::any,0>(const 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::vector<unsigned char,std::allocator<unsigned char> > > &,std::any &) from_json.hpp:118
nlohmann::detail::from_json_fn::operator()<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::vector<unsigned char,std::allocator<unsigned char> > >,std::any &>(const 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::vector<unsigned char,std::allocator<unsigned char> > > &,std::any &) from_json.hpp:480
nlohmann::adl_serializer<std::any,void>::from_json<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::vector<unsigned char,std::allocator<unsigned char> > > const &,std::any>(const 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::vector<unsigned char,std::allocator<unsigned char> > > &,std::any &) adl_serializer.hpp:25
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::vector<unsigned char,std::allocator<unsigned char> > >::get_impl<std::any,0>(priority_tag<0>) json.hpp:1613
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::vector<unsigned char,std::allocator<unsigned char> > >::get<std::any,std::any>() json.hpp:1756
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::vector<unsigned char,std::allocator<unsigned char> > >::operator<std::any,0> std::any() json.hpp:1900
main() main.cpp:7
invoke_main() 0x00007ff7cb0d12c9
__scrt_common_main_seh() 0x00007ff7cb0d116e
__scrt_common_main() 0x00007ff7cb0d102e
mainCRTStartup(void *) 0x00007ff7cb0d135e
<unknown> 0x00007ffa872d54e0
<unknown> 0x00007ffa87e8485b

### Compiler and operating system

MSVC 19.31.31104 on Windows 11 21H2

### Library version

current develop 15fa6a342af7b51cb51a22599026e01f1d81957b

### Validation

- [X] The bug also occurs if the latest version from the [`develop`](https://github.com/nlohmann/json/tree/develop) branch is used.
- [X] I can successfully [compile and run the unit tests](https://github.com/nlohmann/json#execute-unit-tests).

[falbrechtskirchinger]

Seems to work just fine for me with trunk:
https://godbolt.org/z/jcv3Ejsnd

(GCC 11, Clang trunk, MSVC latest)

Edit: Oh wait. That's a runtime issue. My bad.

So, it's trying to assign the value as if std::any were a string type.

[nlohmann]

Can I close this issue then?

[kotori2]

But I didn't get it…… why std::any is a string type?

[nlohmann]

I cannot reproduce the runtime error.

[falbrechtskirchinger]

I cannot reproduce the runtime error.

https://godbolt.org/z/T8fnsrKPz
This reproduces the issue.

from_json() for string types is called, hence the error.

Also, it only happens in the rvalue ref case and I haven't checked yet what type ends up being stored in the any when I remove the rvalue ref.

Edit: It is indeed a basic_json.

[nlohmann]

Thanks! So this only occurs on MSVC? Very strange. I would need to see the stack trace from the exception and the state of the basic_json value.

[falbrechtskirchinger]

Thanks! So this only occurs on MSVC? Very strange. I would need to see the stack trace from the exception and the state of the basic_json value.

I just tried again with Clang (trunk) and it works just fine with the rvalue ref. a.type() indicates it does contain a basic_json as expected. I can try to step through it with a debugger when built with GCC/Clang to see how it differs from the MSVC stack trace above. But that would have to wait until tomorrow.

[kotori2]

Also, it only happens in the rvalue ref case

It seems on Apple Clang 13.1.6 and gcc 9.4.0 the copy constructor was called. The stack is also different from MSVC.

I would need to see the stack trace from the exception

I've posted the stack trace in the Error messages section.

[falbrechtskirchinger]

I think this can be fixed by conditionally excluding std::any from being implicitly converted. Running it through CI right now.

Edit: I'm excluding it unconditionally, to suppress GCC compile failures with "-Werror=conversion".