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

[Sergei Barannikov 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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s-barannikov wrote:

I agree that this looks suspicious, but this is the only way I could get it compiled without errors.
I would appreciate it if someone could explain to me why this is necessary and how can it be simplified.

https://godbolt.org/z/x6dKhr5x1


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