I run into a compilation error since upgrade to R4.0. Also, this error is generated for some models (most likely the models that use the integrate_1d) function but other models run perfectly fine.

I am using Ubuntu 20.04.1 LTS and R version 4.0.2

This is the error I am getting

```
Compilation ERROR, function(s)/method(s) not created!
Error in compileCode(f, code, language = language, verbose = verbose) :
/usr/local/lib/R/site-library/StanHeaders/include/stan/math/rev/mat/functor/adj_jac_apply.hpp:535:10: required from ‘void stan::math::adj_jac_vari<F, Targs>::chain() [with F = stan::math::internal::softmax_op; Targs = {Eigen::Matrix<stan::math::var, -1, 1, 0, -1, 1>}]’/usr/local/lib/R/site-library/StanHeaders/include/stan/math/rev/mat/functor/adj_jac_apply.hpp:531:8: required from here/usr/local/lib/R/site-library/RcppEigen/include/Eigen/src/Core/DenseCoeffsBase.h:650:34: warning: ignoring attributes on template argument ‘Eigen::internal::packet_traits<double>::type’ {aka ‘__vector(4) double’} [-Wignored-attributes] 650 | return internal::first_aligned<int(unpacket_traits<DefaultPacketType>::alignment),Derived>(m); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~make: *** [/usr/lib/R/etc/Makeconf:176: filea863c55825ef.o] Error 1
```