sort_asc allows NaN input

[syclik]

Summary:

sort_asc() and sort_desc() on a structure that contains NaN does not behave consistently.

• break into following and include each in relevant header
  • prim/arr/fun/sort_asc.hpp
  • prim/arr/fun/sort_desc.hpp
  • prim/mat/fun/sort_asc.hpp
  • prim/mat/fun/sort_desc.hpp
• remove rev and fwd sort files and includes in relevant headers
• throw exception if NaN found

Can leave tests as is.

Reproducible Steps:

There's no ordering imposed for NaN, so no need.

Current Output:

Undefined.

Expected Output:

Throw exception.

Current Version:

v2.11.0

[randommm]

This problem came when we were adding nan tests: stan-dev/stan@369121d#commitcomment-7443051

[syclik]

Thanks for digging that up. What should we do? I would prefer if we checked and failed on nan input.

On Mar 7, 2016, at 11:23 AM, Marco Inacio notifications@github.com wrote:

This problem came when we were adding nan tests: stan-dev/stan@369121d#commitcomment-7443051

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[bob-carpenter]

I think checking and failing on NaN input is the
right thing to do. +/- infinity should be OK. And
we only need a specialized test if the input type is
not an integer.