[llvm] [ADT] Fix llvm::concat_iterator for `ValueT == common_base_class *` (PR #144744)

[Javier Lopez-Gomez via llvm-commits]

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@@ -996,13 +1003,17 @@ class concat_iterator
 
   static constexpr bool ReturnsByValue =
       !(std::is_reference_v<decltype(*std::declval<IterTs>())> && ...);
-
+  static constexpr bool ReturnsConvertibleType =
+      !all_types_equal_v<std::remove_cv_t<ValueT>,
+                         std::remove_cv_t<std::remove_reference_t<
+                             decltype(*std::declval<IterTs>())>>...> &&
+      (std::is_convertible_v<decltype(*std::declval<IterTs>()), ValueT> && ...);
+
+  // Cannot return a reference type if a conversion takes place, provided that
+  // the result of dereferencing all `IterTs...` is convertible to `ValueT`.
   using reference_type =
-      typename std::conditional_t<ReturnsByValue, ValueT, ValueT &>;
-
-  using handle_type =
-      typename std::conditional_t<ReturnsByValue, std::optional<ValueT>,
-                                  ValueT *>;
+      typename std::conditional_t<ReturnsByValue || ReturnsConvertibleType,
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jalopezg-git wrote:

It should not be needed; removed.  PTAL and feel free to approve it it is all good :+1:.

https://github.com/llvm/llvm-project/pull/144744