[STLExtras] Add a default value to drop_begin

[kazu]

Summary

This patch adds the default value of 1 to drop_begin.

In the llvm codebase, 70% of calls to drop_begin have 1 as the second
argument. The interface similar to with std::next should improve
readability.

This patch converts a couple of calls to drop_begin as examples.

[dblaikie]

Sounds good, thanks!

llvm/include/llvm/ADT/STLExtras.h

 /// Returns true if the given container only contains a single element.
 template <typename ContainerTy> bool hasSingleElement(ContainerTy &&C) {
   auto B = std::begin(C), E = std::end(C);
   return B != E && std::next(B) == E;
 }
 
 /// Return a range covering \p RangeOrContainer with the first N elements
 /// excluded.
-template <typename T> auto drop_begin(T &&RangeOrContainer, size_t N) {
+template <typename T> auto drop_begin(T &&RangeOrContainer, size_t N = 1) {
   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,

llvm/lib/Transforms/Utils/LoopUtils.cpp

Optional<MDNode *> llvm::makeFollowupLoopID(
   bool InheritAllAttrs = !InheritOptionsExceptPrefix;
   bool InheritSomeAttrs =
       InheritOptionsExceptPrefix && InheritOptionsExceptPrefix[0] != '\0';
   SmallVector<Metadata *, 8> MDs;
   MDs.push_back(nullptr);
 
   bool Changed = false;
   if (InheritAllAttrs || InheritSomeAttrs) {
-    for (const MDOperand &Existing : drop_begin(OrigLoopID->operands(), 1)) {
+    for (const MDOperand &Existing : drop_begin(OrigLoopID->operands())) {
       MDNode *Op = cast<MDNode>(Existing.get());
 
       auto InheritThisAttribute = [InheritSomeAttrs,
                                    InheritOptionsExceptPrefix](MDNode *Op) {
         if (!InheritSomeAttrs)
           return false;
 
         // Skip malformatted attribute metadata nodes.
Optional<MDNode *> llvm::makeFollowupLoopID(
 
   bool HasAnyFollowup = false;
   for (StringRef OptionName : FollowupOptions) {
     MDNode *FollowupNode = findOptionMDForLoopID(OrigLoopID, OptionName);
     if (!FollowupNode)
       continue;
 
     HasAnyFollowup = true;
-    for (const MDOperand &Option : drop_begin(FollowupNode->operands(), 1)) {
+    for (const MDOperand &Option : drop_begin(FollowupNode->operands())) {
       MDs.push_back(Option.get());
       Changed = true;
     }
   }
 
   // Attributes of the followup loop not specified explicity, so signal to the
   // transformation pass to add suitable attributes.
   if (!AlwaysNew && !HasAnyFollowup)

llvm/unittests/ADT/STLExtrasTest.cpp

for (int n = 0; n < 5; ++n) {
     for (auto &v : drop_begin(vec, n)) {
       EXPECT_EQ(v, i);
       i += 1;
     }
     EXPECT_EQ(i, 5);
   }
 }
 
+TEST(STLExtrasTest, DropBeginDefaultTest) {
+  SmallVector<int, 5> vec{0, 1, 2, 3, 4};
+
+  int i = 1;
+  for (auto &v : drop_begin(vec)) {
+    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);