compile error on Xcode 16.1

[ovaar]

Description

platform: Macos (Sequoia, 15.3.2 (24D81)), Intel x86_64
arch: x86_64
compiler: Xcode Version 16.1 (16B40) apple-clang++
cppstd: 23

I faced the following compile error, when including the #include <nlohmann/json.hpp>

I boiled it down to the [std::enable_if](https://en.cppreference.com/w/cpp/types/enable_if) compiler conditional, but I couldn't figure out the real fix, see bool operator==(const IterImpl& other) const

On top of that I fixed the following compile errors where, apparently, the compiler has become more strict or couldn't match, which I created a PR for #4712

Reproduction steps

* Include nlohmann/json.hpp header
* build

Minimal code example

#include <nlohmann/json.hpp>
const auto json_object = nlohmann::json::parse(std::string());

Error messages

Invalid operands to binary expression ('iterator' (aka 'iter_impl<nlohmann::basic_json<std::map, std::vector, std::string, bool, long long, unsigned long long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char>>, void>>') and 'iterator' (aka 'iter_impl<nlohmann::basic_json<std::map, std::vector, std::string, bool, long long, unsigned long long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char>>, void>>'))

Compiler and operating system

Macos (Sequoia, 15.3.2 (24D81)), Intel x86_64, Xcode Version 16.1 (16B40) apple-clang++

Library version

3.11.3

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]

We use Xcode 16.1 among others (see https://json.nlohmann.me/community/quality_assurance/ for all compilers) in the CI without problems. I could also build the whole test suite in Xcode 16.3 with the C++ standard set to 23.