C++17 <functional> template parameter deductions not working on Xcode 10.1

I've been playing around with Template Argument Deduction with C++17 (and onwards), and was trying to compile this exact sample from cppreference.com

#include <functional>

int func(double) { return 0; }
int main() {
  std::function f{func}; // guide #1 deduces function<int(double)>
  int i = 5;
  std::function g = [&](double) { return i; }; // guide #2 deduces function<int(double)>
}

It compiles and runs fine in their web based compiler they have on that page, but when I try and compile this on my Macbook Pro, it fails, saying

error: no viable constructor or deduction guide for deduction of template arguments of 'function'

I've tried it both in an Xcode (v10.1) project set to C++17, and just running clang directly with -std=c++17.

I've also compiled the above example on different online compilers, such as here: https://godbolt.org/z/ERliha

I've also verified that type deductions for std::pair work, so I can't tell if:

1. I screwed up my toolchains somehow when I was messing with cross compilers.
2. Deduction Guides for this class are missing in Apple's Toolchain.
3. I'm running an old toolchain (I don't know how to check this)
4. ... (Any other reason)