Google's Maile Ohye and then confirmed by Gary Illyes of Google said that if you have a desktop site but no mobile version, but you do have AMP versions - in the mobile-first index world, Google will include the desktop version in their mobile-first index, not the AMP version. Got that, AMP is mobile-friendly but Google has decided, at this point in time, to index the desktop version over the AMP version.
This is a bit shocking, at least to me, espesially after I covered a tweet from Google's Gary Illyes earlier about making sure your primary content is in AMP for this reason. This was the tweet:
@AlanBleiweiss AMP is mobile friendly, so amp— Gary Illyes (@methode) November 13, 2016
Then Jennifer Slegg reported that Gary's colleague, Maile Ohye said the opposite at the State of Search conference, saying they index the desktop version.
I asked Gary about it last night and he said he should have said that:
The point about a site going AMP ready and not having also a mobile version is important. I can't imagine a site, outside of the AMP blog which is fully AMP, that has a desktop version and does not have an mobile version when they implemented an AMP version. So this is probably a rare case that is just theoretical, at best.
Forum discussion at Twitter.