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I'm confused, I thought that there already were 3 around? Or at least 2? gcc plus the one from dlang?

the 3rd, LDC, uses LLVM as backend. Both all share the same frontend. https://dlang.org/download.html

open-D is not a 4th implementation it's a fork.

There also SDC which is an experimental D compiler not using DMD code. I don't know how complete it is but it is actively updated.