requirement: require instances to use a subclass

Library/Homebrew/requirement.rb

@@ -20,6 +20,10 @@ class Requirement
   attr_reader :tags, :name, :cask, :download
 
   def initialize(tags = [])
+    # Only allow instances of subclasses. This base class enforces no constraints on its own.
+    # Individual subclasses use the `satisfy` DSL to define those constraints.
+    raise "Do not call `Requirement.new' directly without a subclass." unless self.class < Requirement
+
     @cask = self.class.cask
     @download = self.class.download
     tags.each do |tag|

Library/Homebrew/test/requirement_spec.rb

@@ -12,7 +12,7 @@ describe Requirement do
   let(:klass) { Class.new(described_class) }
 
   describe "#tags" do
-    subject { described_class.new(tags) }
+    subject { klass.new(tags) }
 
     context "with a single tag" do
       let(:tags) { ["bar"] }
@@ -149,7 +149,7 @@ describe Requirement do
 
   describe "#build?" do
     context "when the :build tag is specified" do
-      subject { described_class.new([:build]) }
+      subject { klass.new([:build]) }
 
       it { is_expected.to be_a_build_requirement }
     end
@@ -186,24 +186,24 @@ describe Requirement do
   end
 
   describe "#eql? and #==" do
-    subject(:requirement) { described_class.new }
+    subject(:requirement) { klass.new }
 
     it "returns true if the names and tags are equal" do
-      other = described_class.new
+      other = klass.new
 
       expect(requirement).to eql(other)
       expect(requirement).to eq(other)
     end
 
     it "returns false if names differ" do
-      other = described_class.new
+      other = klass.new
       allow(other).to receive(:name).and_return("foo")
       expect(requirement).not_to eql(other)
       expect(requirement).not_to eq(other)
     end
 
     it "returns false if tags differ" do
-      other = described_class.new([:optional])
+      other = klass.new([:optional])
 
       expect(requirement).not_to eql(other)
       expect(requirement).not_to eq(other)
@@ -211,21 +211,21 @@ describe Requirement do
   end
 
   describe "#hash" do
-    subject(:requirement) { described_class.new }
+    subject(:requirement) { klass.new }
 
     it "is equal if names and tags are equal" do
-      other = described_class.new
+      other = klass.new
       expect(requirement.hash).to eq(other.hash)
     end
 
     it "differs if names differ" do
-      other = described_class.new
+      other = klass.new
       allow(other).to receive(:name).and_return("foo")
       expect(requirement.hash).not_to eq(other.hash)
     end
 
     it "differs if tags differ" do
-      other = described_class.new([:optional])
+      other = klass.new([:optional])
       expect(requirement.hash).not_to eq(other.hash)
     end
   end

Library/Homebrew/test/requirements_spec.rb

@@ -12,7 +12,8 @@ describe Requirements do
     end
 
     it "merges duplicate requirements" do
-      requirements << Requirement.new << Requirement.new
+      klass = Class.new(Requirement)
+      requirements << klass.new << klass.new
       expect(requirements.count).to eq(1)
     end
   end