Fix expose_stan_functions under Stanc 2.31

[andrjohns]

Part of the expose_stan_functions_hacks code was replacing = nullptr with = 0, intended for the pstream__ = nullptr default argument. However this causes a compilation failure under 2.31, since the generated code now includes default template parameters with nullptr which are then replaced.

This PR updates the regex to only replace the nullptr if it's followed by a parenthesis - as the function definitions will always generate the pstream__ = nullptr as the last argument. I've also added a basic test.

This will also need to be backported to 2.26

[andrjohns]

I've also verified that this fixes the test failure for the prophet package under 2.31

[hsbadr]

Is there other patterns that we need to support here, such as "nullptr,", ",nullptr", or ", nullptr"?

[andrjohns]

I would have said no, but the test failure under ubuntu is making me question that. I'll investigate and update the PR

[hsbadr]

Does this fix #954?

[andrjohns]

It's bizarre, I can't replicate the test failures locally under MacOS, Ubuntu, or Windows

[andrjohns]

Narrowed it down a bit more. The test will fail when run through rcmdcheck() but not when run through test_file()

[andrjohns]

Ah, it looks like sourceCpp-based functions aren't compatible with testthat, since it modifies include paths: https://stat.ethz.ch/pipermail/r-package-devel/2017q3/001951.html

May have to just remove the test entirely (after testing locally)

[hsbadr]

it looks like sourceCpp-based functions aren't compatible with testthat,

I see. Could probably use Catch: https://testthat.r-lib.org/reference/use_catch.html.

Anyway, it works on my local machine.

[hsbadr]

LGTM