[ADT] drop_begin: use adl_begin/adl_end. NFC.

[Meinersbur]

Summary

The instantiation of the drop_begin function template usually fails because the functions begin() and end() do not exist. Only when using on a container from the std namespace (or llvm::iterator_ranges of something derived from std::iterator), they are matched to std::begin() and std::end() due to Koenig-lookup.

Explicitly use llvm::adl_begin and llvm::adl_end to make drop_begin applicable to anything iterable (including C-style arrays).

A solution for general llvm::iterator_ranges was already tried in r244620, but got reverted in r244621 due to MSVC not liking it.

[Meinersbur]

include/llvm/ADT/iterator_range.h

62–65

Space was added by clang-format.

[dblaikie]

I think we now have adl_begin/adl_end helpers in the llvm namespace that
should do the right thigh (which is that the call should be made like
"using std::begin; ...begin(X)..." The same way swap is meant to be called
(this allows a default implementation in the std namespace and allows for
types to implement their own custom implementation in their own namespace
to be found via ADL))

[Meinersbur]

Interesting, I didn't know about adl_begin/adl_end/adl_swap. Thanks. Will update the diff soon.

[Meinersbur]

• Use adl_begin/adl_end

[dblaikie]

Looks good - thanks!

[aaron.ballman]

LGTM!

include/llvm/ADT/iterator_range.h

 template <class T> iterator_range<T> make_range(T x, T y) {
   return iterator_range<T>(std::move(x), std::move(y));
 }
 
 template <typename T> iterator_range<T> make_range(std::pair<T, T> p) {
   return iterator_range<T>(std::move(p.first), std::move(p.second));
 }
 
-template<typename T>
-iterator_range<decltype(begin(std::declval<T>()))> drop_begin(T &&t, int n) {
-  return make_range(std::next(begin(t), n), end(t));
+template <typename T>
+iterator_range<decltype(adl_begin(std::declval<T>()))> drop_begin(T &&t,
+                                                                  int n) {
+  return make_range(std::next(adl_begin(t), n), adl_end(t));
 }
 }
 
 #endif

unittests/ADT/IteratorTest.cpp

 TEST(RangeTest, Distance) {
   std::vector<int> v1;
   std::vector<int> v2{1, 2, 3};
 
   EXPECT_EQ(std::distance(v1.begin(), v1.end()), size(v1));
   EXPECT_EQ(std::distance(v2.begin(), v2.end()), size(v2));
 }
 
+TEST(IteratorRangeTest, DropBegin) {
+  SmallVector<int, 5> vec{0, 1, 2, 3, 4};
+
+  for (int n = 0; n < 5; ++n) {
+    int i = n;
+    for (auto &v : drop_begin(vec, n)) {
+      EXPECT_EQ(v, i);
+      i += 1;
+    }
+    EXPECT_EQ(i, 5);
+  }
+}
+
 } // anonymous namespace