ADT/STLExtras: Introduce llvm::empty(); NFC

[MatzeB]

Summary

This is modeled after C++17 std::empty().

(I got talked into shaving this yak in D53885)

[dblaikie]

(maybe some unit tests?)

Also, what about having only a single implementation:

template <typename Range>
constexpr bool empty(const Range &r) {
  return adl_begin(r) == adl_end(r);
}

Not sure why the standard library doesn't do it this way, but there are probably good reasons... though maybe they don't apply to us/llvm?

[MatzeB]

In D53909#1281525, @dblaikie wrote:

(maybe some unit tests?)

Also, what about having only a single implementation:

template <typename Range>
constexpr bool empty(const Range &r) {
  return adl_begin(r) == adl_end(r);
}

Not sure why the standard library doesn't do it this way, but there are probably good reasons... though maybe they don't apply to us/llvm?

What if the container has an optimized empty() method that works faster than actually producing begin and end iterators?

[MatzeB]

• Add a simple unittest
• Go with the adl_begin() == adl_end() implementation for now. We can always switch to more fine grained overloading when we actually deem it necessary for performance. And after all we will probably just replace it with std::empty() anyway once we use C++17 in llvm.

[dblaikie]

Sure - let's go with your original solution (plus the test case) for forwards-compatibility with the standard.

[dblaikie]

(oh, if you are going to switch back to the original implementation - probably put "empty" for iterator_range in iterator_range, rather than as a non-member)