Fix iterator_adaptor_base/enumerator_iter to allow composition of llvm::enumerate with llvm::make_filter_range

* Properly specify reference type in enumerator_iter
* Fix constness of iterator_adaptor_base::operator*

Differential Revision: https://reviews.llvm.org/D112981

Author: joker-eph

llvm/include/llvm/ADT/STLExtras.h

@@ -1907,8 +1907,7 @@ class enumerator_iter
     : public iterator_facade_base<
           enumerator_iter<R>, std::forward_iterator_tag, result_pair<R>,
           typename std::iterator_traits<IterOfRange<R>>::difference_type,
-          typename std::iterator_traits<IterOfRange<R>>::pointer,
-          typename std::iterator_traits<IterOfRange<R>>::reference> {
+          typename std::iterator_traits<IterOfRange<R>>::pointer> {
   using result_type = result_pair<R>;
 
 public:

llvm/include/llvm/ADT/iterator.h

@@ -296,7 +296,8 @@ class iterator_adaptor_base
     return LHS.I < RHS.I;
   }
 
-  ReferenceT operator*() const { return *I; }
+  const ReferenceT operator*() const { return *I; }
+  ReferenceT operator*() { return *I; }
 };
 
 /// An iterator type that allows iterating over the pointees via some

llvm/unittests/ADT/IteratorTest.cpp

@@ -178,6 +178,16 @@ TEST(FilterIteratorTest, Lambda) {
   EXPECT_EQ((SmallVector<int, 3>{1, 3, 5}), Actual);
 }
 
+TEST(FilterIteratorTest, Enumerate) {
+  auto IsOdd = [](auto N) { return N.value() % 2 == 1; };
+  int A[] = {0, 1, 2, 3, 4, 5, 6};
+  auto Enumerate = llvm::enumerate(A);
+  SmallVector<int> Actual;
+  for (auto IndexedValue : make_filter_range(Enumerate, IsOdd))
+    Actual.push_back(IndexedValue.value());
+  EXPECT_EQ((SmallVector<int, 3>{1, 3, 5}), Actual);
+}
+
 TEST(FilterIteratorTest, CallableObject) {
   int Counter = 0;
   struct Callable {