Stan Math functions that are meant to be exposed in the Stan language have 2 expressions requirements:

• they have to be able to accept Eigen expressions
• they have to work with the expressions efficiently (not evaluate them multiple times)

This is not really documented or expressed clearly for Stan Math developers I think. We need to mention this somewhere more prominently.

It should also mention that you can test expressions for a function not currently exposed in Stan by running:

python3 runTests.py test/expressions/ --only-function "foo(vector) => vector"