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

[David Blaikie via llvm-commits]

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@@ -1032,13 +1032,17 @@ class concat_iterator
 
   static constexpr bool ReturnsByValue =
       !(std::is_reference_v<decltype(*std::declval<IterTs>())> && ...);
+  static constexpr bool ReturnsConvertiblePointer =
+      std::is_pointer_v<ValueT> &&
+      (std::is_convertible_v<decltype(*std::declval<IterTs>()), ValueT> && ...);
 
   using reference_type =
-      typename std::conditional_t<ReturnsByValue, ValueT, ValueT &>;
+      typename std::conditional_t<ReturnsByValue || ReturnsConvertiblePointer,
+                                  ValueT, ValueT &>;
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dwblaikie wrote:

some test coverage would be good either way - to demonstrate what breaks if such changes were to occur (ie: assignability of a dereference expression (`*concat_iter = other_ptr`) would fail to compile if the reference type changed from `T*&` to `T*`)

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