ambiguous fvar template instantiation of ad_promotable with g++ 5.4.0 #528

Summary:

The unit tests are failing with g++ 5.4.0

Description:

I'm not sure what's happening, honestly. I'm not familiar with the code that is breaking. This just came up when trying to merge some new code into Stan. (see http://discourse.mc-stan.org/t/stans-runtests-py-src-test-unit-failing-with-g-works-with-clang/351, stan-dev/stan#2285)

Reproducible Steps:

In Ubuntu 16.04 with g++ 5.4.0 (the default compiler)

make test/unit/math/mix/mat/prob/categorical_test (in the math repo)

Should fail

Current Output:

https://gist.github.com/bbbales2/0a2456d516bbe1200a4d1ee76b936656

Expected Output:

The tests to pass

Additional Information:

I had a dig around. I'm not sure what I'm doing (so don't interpret this as an actual fix), but this change seems to get rid of the error:
bbbales2@52de78f

It seemed like the compiler couldn't decide between using:

template <typename T>
    struct ad_promotable<T, T> {
      enum { value = true };
    };

with T = stan::math::fvarstan::math::var

and

template <typename V, typename T>
    struct ad_promotable<V, fvar<T> > {
      enum { value = ad_promotable<V, T>::value };
    };

with V = stan::math::fvarstan::math::var, T = double

So I just made a type to specifically absorb two fvars. Maybe this is helpful in figuring out the problem. Again I'm not sure what I'm doing so don't interpret this as a fix.

Current Version:

v2.15.0