Bugfix - pow(Eigen, Eigen) forcing Matrix return

[andrjohns]

Summary

As identified here the changes to pow introduced by #2546 have broken the extended distribution tests by forcing the return to always be an Eigen::Matrix input, even when Eigen::Array inputs have been passed.

This PR updates the return type deduction to correctly return Eigen::Array inputs where appropriate.

Tests

Extended distribution tests have been temporarily enabled to ensure that this fixes the issue

Side Effects

N/A

Release notes

Fix bug with vectorised pow() incorrectly forcing Eigen::Matrix return instead of Eigen::Array

Checklist

• Math issue N/A

• Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[rok-cesnovar]

Extended distribution tests have been temporarily enabled to ensure that this fixes the issue

This does not have to be hardcoded. You can enable the extended tests buy ticking the "withRowVectors" in Jenkins: https://jenkins.flatironinstitute.org/job/Stan/job/Math/job/PR-2793/build?delay=0sec

I already started them https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FMath/detail/PR-2793/4/pipeline

If all goes green I will approve. Thanks again!

[andrjohns]

Extended distribution tests have been temporarily enabled to ensure that this fixes the issue

This does not have to be hardcoded. You can enable the extended tests buy ticking the "withRowVectors" in Jenkins: https://jenkins.flatironinstitute.org/job/Stan/job/Math/job/PR-2793/build?delay=0sec

I already started them https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FMath/detail/PR-2793/4/pipeline

If all goes green I will approve. Thanks again!

Oh neat, thanks!

[SteveBronder]

Lgtm