Consistent return type for size()

[andrjohns]

Description

As mentioned in #2641, the math::size() overloads should have a return type consistent with Eigen's size() member to avoid compiler errors when calling code such as:

  Eigen::VectorXd a(2);
  a << 1, 2;

  std::max(a.size(), stan::math::size(a));

Which resolves to:

std::max(size_t, Eigen::Index);

Current Version:

v4.4.0

[bob-carpenter]

Thanks, @andrjohns. Is there code that produces compiler errors now? If so, it'd help to say what they are. That may be the std::max call, but I couldn't tell.

The fundamental problem is that Eigen returns signed sizes/indexes and std::vector returns unsigned ones.

I think the problem with std::max is that it can't infer a template parameter if the types don't match. We could solve this by implementing our own max that just works on int types like Stan does.

[andrjohns]

In terms of c++, just the example I posted in the issue. Im not sure if there are other examples or wider impacts, since this a subset of the issues addressed in the linked PR by @SteveBronder - he might have a better idea

[SteveBronder]

I think having our own max for int types would be the way to go. Though I'm not sure how to handle the return for mixes. Maybe just always long int? Cuz then I think size_t is not a problem