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

[Sergei Barannikov via llvm-commits]

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@@ -614,12 +610,11 @@ template <typename T> struct ValueIsPresent<std::optional<T>> {
   static inline decltype(auto) unwrapValue(std::optional<T> &t) { return *t; }
 };
 
-// If something is "nullable" then we just compare it to nullptr to see if it
-// exists.
+// Specialization for types convertible to bool.
 template <typename T>
-struct ValueIsPresent<T, std::enable_if_t<IsNullable<T>>> {
+struct ValueIsPresent<T, std::enable_if_t<std::is_constructible_v<bool, T>>> {
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s-barannikov wrote:

> But this enable_if guards both `isPresent` and `unwrapValue` -- don't we need extra care for the latter?

Maybe, although constructibility from nullptr_t doesn't guarantee that the default implementation of unwrapValue would fit.

As I noted in the other comment, I think it would be best if we provided explicit specializations for each type like we do with `simplify_type`, for example. Possibly provide a generic one for `T *`, too. I would't commit myself to implementing this though, I've already shown my expertise in metaprogramming.


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