[ADT] Use adl_being/end in concat

This is to make sure that ADT helpers consistently use argument
dependent lookup when dealing with input ranges.

This was a part of llvm#87936 but reverted due to buildbot failures.

Author: kuhar

llvm/include/llvm/ADT/STLExtras.h

@@ -1112,8 +1112,8 @@ class concat_iterator
   /// We need the full range to know how to switch between each of the
   /// iterators.
   template <typename... RangeTs>
-  explicit concat_iterator(RangeTs &&... Ranges)
-      : Begins(std::begin(Ranges)...), Ends(std::end(Ranges)...) {}
+  explicit concat_iterator(RangeTs &&...Ranges)
+      : Begins(adl_begin(Ranges)...), Ends(adl_end(Ranges)...) {}
 
   using BaseT::operator++;
 
@@ -1142,26 +1142,25 @@ template <typename ValueT, typename... RangeTs> class concat_range {
 public:
   using iterator =
       concat_iterator<ValueT,
-                      decltype(std::begin(std::declval<RangeTs &>()))...>;
+                      decltype(adl_begin(std::declval<RangeTs &>()))...>;
 
 private:
   std::tuple<RangeTs...> Ranges;
 
-  template <size_t... Ns>
-  iterator begin_impl(std::index_sequence<Ns...>) {
+  template <size_t... Ns> iterator begin_impl(std::index_sequence<Ns...>) {
     return iterator(std::get<Ns>(Ranges)...);
   }
   template <size_t... Ns>
   iterator begin_impl(std::index_sequence<Ns...>) const {
     return iterator(std::get<Ns>(Ranges)...);
   }
   template <size_t... Ns> iterator end_impl(std::index_sequence<Ns...>) {
-    return iterator(make_range(std::end(std::get<Ns>(Ranges)),
-                               std::end(std::get<Ns>(Ranges)))...);
+    return iterator(make_range(adl_end(std::get<Ns>(Ranges)),
+                               adl_end(std::get<Ns>(Ranges)))...);
   }
   template <size_t... Ns> iterator end_impl(std::index_sequence<Ns...>) const {
-    return iterator(make_range(std::end(std::get<Ns>(Ranges)),
-                               std::end(std::get<Ns>(Ranges)))...);
+    return iterator(make_range(adl_end(std::get<Ns>(Ranges)),
+                               adl_end(std::get<Ns>(Ranges)))...);
   }
 
 public:

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -504,6 +504,16 @@ TEST(STLExtrasTest, ConcatRange) {
   EXPECT_EQ(Expected, Test);
 }
 
+TEST(STLExtrasTest, ConcatRangeADL) {
+  // Make sure that we use the `begin`/`end` functions from `some_namespace`,
+  // using ADL.
+  some_namespace::some_struct S0;
+  S0.data = {1, 2};
+  some_namespace::some_struct S1;
+  S1.data = {3, 4};
+  EXPECT_THAT(concat<const int>(S0, S1), ElementsAre(1, 2, 3, 4));
+}
+
 template <typename T> struct Iterator {
   int i = 0;
   T operator*() const { return i; }