use preprocessor guards to error include order problems #383
Description
Summary:
From @jpritikin on the stan-dev group in response to a query about header include order causing problems:
Is there a way to catch this kind of thing with the C preprocessor? For example, a trait could #define I_AM_A_TRAIT and then if some other header got included later in the wrong order then you could #error.
Description:
We run into order dependencies in our header includes, primarily for traits. For example, we often require the Eigen version of a recursive template trait metaprogram to be included before the standard vector version in order ot handle arrays of matrices. We'd like to be able to catch wrong include orders at build time rather than getting perplexing errors.
Reproducible Steps:
A concrete example is including stan/math/prim/arr.hpp before stan/math/prim/mat.hpp.
Current Output:
Compliation failure.
Expected Output:
Prepropcoessor error with informative error message.
Current Version:
v2.11.0