Basically, right, that song 'doing the rounds' purporting to be a collaboration between OneRepublic and Calvin Harris is a big old fake.
The song, 'Walking On The Sun', was attributed to those people above by fans on the www, but as you'll see from the following tweets it's all a big misunderstanding:
FYI to whoever leaked the "@CalvinHarris featuring OneRepublic" song- It ain't me singing. Would gladly collab in the future but it's fake
— OneRepublic (@OneRepublic) May 31, 2016
And Calvin had this to say:
So now we know.
Here's the song in question: