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Snapdragon 845 machines will follow in the second part of the year and this has been confirmed by the company’s executives incluing the EVP and President of QCT Cristiano Amon. It simply takes time from the announced SoC to a fully ready platform for the Windows 10 PC.

You should see Snapdragon 845 in phones first and most likely in PCs for the back-to-school market by early Q3 2018. Qualcomm executives pointed out that this is a big learning curve for both Microsoft and Qualcomm but that both companies are very confident in its success.
Source: Snapdragon 845 arriving for Windows later in 2018
Via Windows 10 PCs Powered by Snapdragon 845 Chip Launching in Second Half of 2018