Here is a thread from a few months ago, but still pretty interesting where Barry Adams, an SEO, posted on Twitter that you will lose traffic when you do an HTTPS migration. Gary Illyes from Google responded “false” and then an SEO and Google fight broke out.
Here is the tweet:

Google treats HTTP to HTTPS as a site move, but you can’t use GSC Change of Address for it. So basically, lose traffic and suck it up. pic.twitter.com/1AUsy8Fhbq
— Barry Adams 📈 (@badams) August 30, 2017
Gary’s response:

False
— Gary “鯨理” Illyes (@methode) August 30, 2017
Barry says give me a citation on how it is false:

Citation needed.
— Barry Adams 📈 (@badams) August 30, 2017
John Mueller jumps in, who is also at Google and says:

Do you want someone from Google to confirm it? 🙂
— John ☆.o(≧▽≦)o.☆ (@JohnMu) August 30, 2017
It goes back and forth more and then John and Gary stop responding.
Google has said this before, that with HTTPS migrations, you shouldn’t see a negative result in your Google traffic. Just make sure to do it right and all at once. Here is also an FAQ around HTTPS migrations.
I’ve done several HTTPS migrations that results in zero Google traffic loss.
Forum discussion at Twitter.
This post was scheduled to go live today but was written earlier – I am currently offline today.