I had same issue too.
Error is fixed by adding () around [UE4Atomic_Private::TIsSupportedSize::Value] , but I don’t know why. And I can’t guarantee it is safe.
template <typename T>
struct alignas((UE4Atomic_Private::TIsSupportedSize<sizeof(T)>::Value) ? alignof(UE4Atomic_Private::TUnderlyingIntegerType_T<T>) : alignof(T)) TAtomicBase_Basic