[ADT] Move drop_begin from iterator_range.h into STLExtras.

[lhames]

Summary

drop_begin depends on adl_begin/adl_end, which are defined in STLExtras.h,
but we can't just #include STLExtras.h in iterator_range.h as that would
introduce a circular reference (STLExtras.h already depends on
iterator_range.h). The simplest solution is to move drop_begin into
STLExtras.h, which is a reasonable home for it anyway.

[dblaikie]

Sounds good - thanks!

llvm/include/llvm/ADT/STLExtras.h

 }
 
 /// Test whether \p RangeOrContainer is empty. Similar to C++17 std::empty.
 template <typename T>
 constexpr bool empty(const T &RangeOrContainer) {
   return adl_begin(RangeOrContainer) == adl_end(RangeOrContainer);
 }
 
+/// Return a range covering \p RangeOrContainer with the first N elements
+/// excluded.
+template <typename T>
+auto drop_begin(T &&RangeOrContainer, size_t N) ->
+    iterator_range<decltype(adl_begin(RangeOrContainer))> {
+  return make_range(std::next(adl_begin(RangeOrContainer), N),
+                    adl_end(RangeOrContainer));
+}
+
 // mapped_iterator - This is a simple iterator adapter that causes a function to
 // be applied whenever operator* is invoked on the iterator.
 
 template <typename ItTy, typename FuncTy,
           typename FuncReturnTy =
             decltype(std::declval<FuncTy>()(*std::declval<ItTy>()))>
 class mapped_iterator
     : public iterator_adaptor_base<

llvm/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(adl_begin(std::declval<T>()))> drop_begin(T &&t,
-                                                                  int n) {
-  return make_range(std::next(adl_begin(t), n), adl_end(t));
-}
 }
 
 #endif

llvm/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

llvm/unittests/ADT/STLExtrasTest.cpp

TEST(STLExtrasTest, EmptyTest) {
   EXPECT_FALSE(llvm::empty(NotE));
 
   auto R0 = make_range(V.begin(), V.begin());
   EXPECT_TRUE(llvm::empty(R0));
   auto R1 = make_range(V.begin(), V.end());
   EXPECT_FALSE(llvm::empty(R1));
 }
 
+TEST(STLExtrasTest, DropBeginTest) {
+  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);
+  }
+}
+
 TEST(STLExtrasTest, EarlyIncrementTest) {
   std::list<int> L = {1, 2, 3, 4};
 
   auto EIR = make_early_inc_range(L);
 
   auto I = EIR.begin();
   auto EI = EIR.end();
   EXPECT_NE(I, EI);