`from_cbor` incompatible with `std::vector<std::byte>` as `binary_t`

[burnpanck]

Description

Trying to use json::from_cbor with a binary_t set to std::vector<std::byte> will fail because the decoder appears to be hard-coded to push_back elements of std::uint8_t.

It appears to be related to #4756 where they report essentially the same issue for from_msgpack; although that one was fixed in #4760 by adding a specialisation to char_traits. It appears that type trait is not being used in the implementation of from_cbor.

Reproduction steps

Attempt to compile the below minimal example.

Expected vs. actual results

Expected to compile.

Minimal code example

#include <nlohmann/json.hpp>

using json = nlohmann::basic_json<std::map,        // ObjectType
                                  std::vector,     // ArrayType
                                  std::string,     // StringType
                                  bool,            // BooleanType
                                  std::int64_t,    // NumberIntegerType
                                  std::uint64_t,   // NumberUnsignedType
                                  double,          // NumberFloatType
                                  std::allocator,  // AllocatorType
                                  nlohmann::adl_serializer,  // JSONSerializer
                                  std::vector<std::byte>,    // BinaryType
                                  void                       // CustomBaseClass
                                  >;

int main(){
    std::vector<char> data;
    auto decoded = json::from_cbor(data);
    return 0;
}

See also on Compiler Explorer.

Error messages

On Clang 20:

/opt/compiler-explorer/libs/nlohmann_json/trunk/single_include/nlohmann/json.hpp:12768:20: error: no matching member function for call to 'push_back'
 12768 |             result.push_back(static_cast<std::uint8_t>(current));

Compiler and operating system

Clang 20, GCC 15, both on Compiler Explorer (i.e. linux?)

Library version

trunk

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[nlohmann]

PTAL at #4805

[burnpanck]

Wow, that was quick! Looks good to me.

[nlohmann]

Unfortunately, that code does not work on macOS or Windows. I need to investigate - any hints welcome!

[burnpanck]

I just left a comment; I believe the test-case itself may be flawed. You seem to have re-used my minimal code example as a regression test; that one was correctly highlighting that the code does not compile - but it's unclear if it should run successfully. In my understanding, when viewing a byte-stream as a sequence of CBOR items, the empty byte-string used in that example is valid; it decodes to zero CBOR items. However from_cbor attempts to decode a single CBOR item, for which the empty byte-string is probably correctly determined as invalid data. Instead, perhaps use {0xa0}; the correct CBOR encoding of an empty map/object.