nanort | NanoRT , single header only modern ray tracing kernel | GPU library

Replace std::allocator::pointer with T* and std::allocator::size_type with std::size_t. That is what those deprecated member type aliases are defined as.

is this a few-line fix, or is this much more complicated than that and would require restructuring large portions of the code?

This seems like a trivial change.

The error suggests using std::allocator_traits

std::allocator_traits should be used if the allocator type is templated. That appears to not be the case here, and so using std::allocator_traits would be unnecessary.

But if we were to fix the problem in a case where the allocator is templated: Let's say the template argument is Alloc. Then you should replace Alloc::pointer with std::allocator_traits::pointer and Alloc::size_type with std::allocator_traits::size_type.

However, it looks like part of this code does not work in C++17, as it uses something that has become deprecated

Actually, use of deprecated features still works. It should still be fixed though since it will stop working in C++20. std::allocator::pointer has been removed in C++20.