[llvm] [ADT] Make concat able to handle ranges with iterators that return by value (such as zip) (PR #112783)

[Jakub Kuderski via llvm-commits]

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@@ -1065,27 +1075,30 @@ class concat_iterator
   /// Returns null if the specified iterator is at the end. Otherwise,
   /// dereferences the iterator and returns the address of the resulting
   /// reference.
-  template <size_t Index> ValueT *getHelper() const {
+  template <size_t Index> handle_type getHelper() const {
     auto &Begin = std::get<Index>(Begins);
     auto &End = std::get<Index>(Ends);
     if (Begin == End)
-      return nullptr;
+      return {};
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kuhar wrote:

Do we need this check at all? Should this be turned into an assertion instead or pushed to the callees?

https://github.com/llvm/llvm-project/pull/112783