[llvm] [ADT] Add detection utility for incomplete types (PR #65495)

[Jakub Kuderski via llvm-commits]

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@@ -2476,6 +2476,16 @@ bool hasNItemsOrLess(ContainerTy &&C, unsigned N) {
 template <class Ptr> auto to_address(const Ptr &P) { return P.operator->(); }
 template <class T> constexpr T *to_address(T *P) { return P; }
 
+// Detect incomplete types, relying on the fact that their size is unknown.
+namespace detail {
+template <typename T> using has_sizeof = decltype(sizeof(T));
+} // namespace detail
+
+/// Detects when type `T` is incomplete. This is true for forward declarations
+/// and types with a full definition.
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kuhar wrote:

Right, thanks!

https://github.com/llvm/llvm-project/pull/65495