I have addressed the Baxalta / Shire transaction here:
https://ttlc.intuit.com/questions/3814587-can-you-help-baxalta-shire-merger-cash-and-stock
The transaction is fully taxable and you use the sum of cash plus
FMV of the stock as "proceeds" to determine your reportable gain or
loss. It's "as if" all the proceeds were paid to you in cash and then
you used some of the cash to buy Shire stock. Your basis in the Shire stock is the same as the stock's per share FMV used in the "proceeds" calculation and your holding period begins at that time.
Tom Young
