rebar3: enable compatibility with multiple Erlang versions

`rebar3` is designed to work with multiple versions of Erlang. However,
it can only be used with versions of Erlang that are at least as new as
the one it was built with.

To maximise compatibility with supported versions of Erlang, let's build
with `erlang@25`. We also adjust the `install` method to ensure that
we're always building with the oldest supported version of Erlang.

Formula/r/rebar3.rb

@@ -21,9 +21,23 @@ class Rebar3 < Formula
     sha256 cellar: :any_skip_relocation, x86_64_linux:  "3049b6e29c6f38ac6ba4bbdfbdfd65e5a08e978b739957601c02aa6791cea5a5"
   end
 
+  depends_on "erlang@25" => [:build, :test]
   depends_on "erlang"
 
   def install
+    erlang_build_dep = deps.find { |dep| dep.build? && dep.name.match?(/^erlang@\d+$/) }&.to_formula
+    odie "Could not find build-time erlang!" if erlang_build_dep.blank?
+
+    # To guarantee compatibility with various erlang versions, build with an older erlang.
+    # We want to use `erlang@#{x-2}` where x is the major version of the `erlang` formula.
+    build_erlang_version = erlang_build_dep.version.major.to_i
+    wanted_erlang_version = Formula["erlang"].version.major.to_i - 2
+    if wanted_erlang_version != build_erlang_version
+      odie "This formula should be built with `erlang@#{wanted_erlang_version}`"
+    end
+
+    # Ensure we're building with versioned `erlang`
+    ENV.remove "PATH", "#{Formula["erlang"].opt_bin}:"
     system "./bootstrap"
     bin.install "rebar3"
 
@@ -33,6 +47,15 @@ def install
   end
 
   test do
-    system bin/"rebar3", "--version"
+    deps.each do |dep|
+      next unless dep.name.match?(/^erlang(@\d+)?$/)
+
+      erlang = dep.to_formula
+      erlang_bin = erlang.opt_bin
+      erlang_version = erlang.version.major
+      with_env(PATH: "#{erlang_bin}:#{ENV["PATH"]}") do
+        assert_match "OTP #{erlang_version}", shell_output("#{bin}/rebar3 --version")
+      end
+    end
   end
 end