Merge pull request #11840 from gottesmm/pr-f328c5f39fd98eb366086253b10e0d6bd653d517

Author: swift-ci

include/swift/Basic/STLExtras.h

@@ -219,17 +219,33 @@ inline void set_union_for_each(const Container1 &C1, const Container2 &C2,
   set_union_for_each(C1.begin(), C1.end(), C2.begin(), C2.end(), f);
 }
 
+/// If \p it is equal to \p end, then return \p defaultIter. Otherwise, return
+/// std::next(\p it).
+template <typename Iterator>
+inline Iterator next_or_default(Iterator it, Iterator end,
+                                Iterator defaultIter) {
+  return (it == end) ? defaultIter : std::next(it);
+}
+
+/// If \p it is equal to \p begin, then return \p defaultIter. Otherwise, return
+/// std::prev(\p it).
+template <typename Iterator>
+inline Iterator prev_or_default(Iterator it, Iterator begin,
+                                Iterator defaultIter) {
+  return (it == begin) ? defaultIter : std::prev(it);
+}
+
 /// Takes an iterator and an iterator pointing to the end of the iterator range.
 /// If the iterator already points to the end of its range, simply return it,
 /// otherwise return the next element.
 template <typename Iterator>
 inline Iterator next_or_end(Iterator it, Iterator end) {
-  return (it == end) ? end : std::next(it);
+  return next_or_default(it, end, end);
 }
 
 template <typename Iterator>
 inline Iterator prev_or_begin(Iterator it, Iterator begin) {
-  return (it == begin) ? begin : std::prev(it);
+  return prev_or_default(it, begin, begin);
 }
 
 /// @}