Add tests and fix type signature

Library/Homebrew/dev-cmd/release-notes.rb

@@ -32,6 +32,9 @@ module Homebrew
   def release_notes
     args = release_notes_args.parse
 
+    # TODO: (2.8) Deprecate this command now that the `brew release` command exists.
+    # odeprecated "`brew release-notes`"
+
     previous_tag = args.named.first
 
     if previous_tag.present?
@@ -56,7 +59,7 @@ module Homebrew
       odie "Ref #{ref} does not exist!"
     end
 
-    release_notes = ReleaseNotes.generate_release_notes previous_tag, end_ref, markdown: T.must(args.markdown?)
+    release_notes = ReleaseNotes.generate_release_notes previous_tag, end_ref, markdown: args.markdown?
 
     $stderr.puts "Release notes between #{previous_tag} and #{end_ref}:"
     puts release_notes

Library/Homebrew/release_notes.rb

@@ -10,7 +10,7 @@ module ReleaseNotes
   module_function
 
   sig {
-    params(start_ref: T.any(String, Version), end_ref: T.any(String, Version), markdown: T::Boolean)
+    params(start_ref: T.any(String, Version), end_ref: T.any(String, Version), markdown: T.nilable(T::Boolean))
       .returns(String)
   }
   def generate_release_notes(start_ref, end_ref, markdown: false)

Library/Homebrew/test/dev-cmd/release_spec.rb

@@ -0,0 +1,8 @@
+# typed: false
+# frozen_string_literal: true
+
+require "cmd/shared_examples/args_parse"
+
+describe "Homebrew.release_args" do
+  it_behaves_like "parseable arguments"
+end

Library/Homebrew/test/release_notes_spec.rb

@@ -0,0 +1,33 @@
+# typed: false
+# frozen_string_literal: true
+
+require "release_notes"
+
+describe ReleaseNotes do
+  before do
+    HOMEBREW_REPOSITORY.cd do
+      system "git", "init"
+      system "git", "commit", "--allow-empty", "-m", "Initial commit"
+      system "git", "tag", "release-notes-testing"
+      system "git", "commit", "--allow-empty", "-m", "Merge pull request #1 from Homebrew/fix", "-m", "Do something"
+      system "git", "commit", "--allow-empty", "-m", "make a change"
+      system "git", "commit", "--allow-empty", "-m", "Merge pull request #2 from User/fix", "-m", "Do something else"
+    end
+  end
+
+  describe ".generate_release_notes" do
+    it "generates release notes" do
+      expect(described_class.generate_release_notes("release-notes-testing", "HEAD")).to eq <<~NOTES
+        https://github.com/Homebrew/brew/pull/2 (@User) - Do something else
+        https://github.com/Homebrew/brew/pull/1 (@Homebrew) - Do something
+      NOTES
+    end
+
+    it "generates markdown release notes" do
+      expect(described_class.generate_release_notes("release-notes-testing", "HEAD", markdown: true)).to eq <<~NOTES
+        - [Do something else](https://github.com/Homebrew/brew/pull/2) (@User)
+        - [Do something](https://github.com/Homebrew/brew/pull/1) (@Homebrew)
+      NOTES
+    end
+  end
+end