You can follow my development even further—the lingering question in that gist is how to expand the guard code to see if it is valid in a guard.

I tried my hand at a pure-Elixir guard expansion in an earlier PR, documenting my thoughts as I went: https://github.com/elixir-lang/elixir/pull/5854

After José gave me some direction in response to that, I produced the ultimate PR: https://github.com/elixir-lang/elixir/pull/5857

It simply relies on calling the :elixir_expand.expand function that the compiler uses, with a guard context—so the defguard implementation is always up-to-date with what the compiler allows in guards. Waaay more elegant than munging AST myself.

Funnily enough, the tests I wrote for defguard actually uncovered some missing assertions in that function. Some invalid expressions would make it past that step and return a different error much deeper in. It's kinda cool I indirectly contributed to the compiler.

I took on the feature mostly because over 4 years ago I'd made a gist to do exactly that, minus the validation, and when I found the issue on github for it, it gave me a lot of pleasant closure to get it merged in! https://gist.github.com/christhekeele/8284977/revisions#diff...