Go to their website. Under shareholders/investors there should be a tax information for shareholders due to the merger topic which will explain the taxability or not and provide worksheets on how to compute.

I do have the K1 paperwork.  However it is not real clear.  Each partnership unit is swapped for .58 common stock in the new corporation.  The partner is not getting cash.  But it is not the classic stock for stock merger that has no tax consequences.

Wasn't Cedar Fair a stock company, many years ago?  I think I had a client who had claimed it as worthless, and then was surprised to start getting dividend checks again. 

In any case, here is what one source says:

Will the merger transaction be a taxable event?

Generally not, however, there are certain circumstances described in further detail in the Form S-4
registration statement form CopperSteel HoldCo., Inc., including in the sections entitled “Summary—
Material U.S Federal Income Tax Consequences of the Mergers to Cedar Fair” and “Material U.S. Federal Income Tax Consequences for Cedar Fair Unitholders”, and in Section 8.8(a) (Tax Treatment) of the merger agreement, that may result in tax consequences to unitholders upon closing. Please contact your qualified tax professional for guidance.

I have a partner that has a negative tax basis.  I can't find any information on this treatment but I'm leaning toward the recognizing of a capital gain for the negative basis to bring it back up to zero.

My other thought was recognizing nothing & carrying the negative basis over to the stock obtained in the merger.  

Thoughts?

There's no such thing as negative basis. It can only go to zero.

Per the supplemental Merger info they are reporting his unified Tax Basis as negative.  His capital account was negative at the beginning of the year. 

You can have a negative capital account.

I think you have a Capital Account, computed on a Tax Basis, that is negative. That is not his basis.

Is this like stock in a corporation that makes nontaxable distributions?  Were the "partners" receiving distributions that reduced their capital account below zero?  Were they reporting this as capital gain?  Or could the internal capital account be lower than the external cost basis?  

I believe you are correct to recognize the capital gain on the negative basis.