Clang from Homebrew is a good option (or compile one by yourself).
You may also look at the features that have been incorporated on Clang's official website.
❯ clang++ --version
Homebrew clang version 13.0.0
Target: arm64-apple-darwin21.1.0
Thread model: posix
InstalledDir: /opt/homebrew/opt/llvm/bin
❯ cat test.cpp
#include <concepts>
#include <iterator>
static_assert(std::convertible_to<char, int>);
static_assert(std::forward_iterator<char*>);
❯ clang++ -std=c++20 -c test.cpp
❯