How to start with Swift Compiler

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What should I learn if I want to contribute to the Swift compiler?

I've made it my goal to be able to contribute to the Swift compiler by the end of 2019. For some background, most of my software development experience is in iOS. I've never worked on a compiler, and I haven't touched C++ since a data structures and algorithms course I took in college 3 years ago. Is there a particular part of C++ that would be handy to know? Are there things about compiler development in particular I ought to study? Thanks!

Lots of good stuff in this thread!