[llvm] [ADT] Specialize ValueIsPresent for PointerUnion (PR #121847)

[Jakub Kuderski via llvm-commits]

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@@ -259,6 +259,17 @@ struct CastInfo<To, const PointerUnion<PTs...>>
                                           CastInfo<To, PointerUnion<PTs...>>> {
 };
 
+// The default implementation of isPresent() for nullable types returns true
+// if the active member is not the first one, even if its value is nullptr.
+// Override the default behavior to return false for all possible null values.
+template <typename... PTs>
+struct ValueIsPresent<PointerUnion<PTs...>,
+                      std::enable_if_t<IsNullable<PointerUnion<PTs...>>>> {
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kuhar wrote:

Actually, I think something simple like this should work:
* create a new free function `isNull` in `Casting.h` and use it in `isPresent`
* create an overload for PointerUnion types in `PointerUnion.h`

https://github.com/llvm/llvm-project/pull/121847