msgpack parser failed to parse null as Map key

What is the issue you have?

I'm trying to parse msgpack data like:

{
    null: 1
}

which serializes as 81 C0 01. But nlohmann::json::from_msgpack failed to parse it with

[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack string: expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0xC0

Please describe the steps to reproduce the issue.

↓

Can you provide a small but working code example?

#include <nlohmann/json.hpp>
int main() {
    const uint8_t src[] = {0x81, 0xC0, 0x01};
    nlohmann::json j = nlohmann::json::from_msgpack(src);
}

What is the expected behavior?

From the msgpack specs it doesn't restricts which value could be used on Map keys

And what is the actual behavior instead?

The specs above says

applications may remove Binary type, restrict keys of map objects to be String type, and put some restrictions to make the semantics compatible with JSON

So maybe it could be serialized to

{
    "null": 1
}

like other implementation does

Which compiler and operating system are you using?

• Compiler: MSVC 19.30.30705
• Operating system: Windows 11 22000.527
  Also reproduced on gcc11

Which version of the library did you use?

• latest release version 3.10.5
• other release - please state the version: 3.10.4
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below
• N/A

Start at the nlohmann::json::from_msgpack entry point and reproduce the issue with the byte sequence 81 C0 01 from the report. Clarify whether null map keys should be accepted or converted for JSON compatibility, then add coverage for the agreed behavior and run the relevant unit tests.