Add #uses_from_macos specs

Library/Homebrew/test/formula_spec.rb

@@ -1387,4 +1387,92 @@ describe Formula do
       end
     end
   end
+
+  describe "#uses_from_macos" do
+    context 'on Linux' do
+      before do
+        allow(OS).to receive(:mac?).and_return(false)
+      end
+
+      it "acts like #depends_on" do
+        f = formula "foo" do
+          url "foo-1.0"
+
+          uses_from_macos("foo")
+        end
+
+        expect(f.class.stable.deps.first.name).to eq("foo")
+        expect(f.class.devel.deps.first.name).to eq("foo")
+        expect(f.class.head.deps.first.name).to eq("foo")
+      end
+
+      it "ignores MacOS specifications" do
+        f = formula "foo" do
+          url "foo-1.0"
+
+          uses_from_macos("foo", after: :mojave)
+        end
+
+        expect(f.class.stable.deps.first.name).to eq("foo")
+        expect(f.class.devel.deps.first.name).to eq("foo")
+        expect(f.class.head.deps.first.name).to eq("foo")
+      end
+    end
+
+    context 'on MacOS' do
+      before do
+        sierra_os_version = OS::Mac::Version.from_symbol(:sierra)
+
+        allow(OS).to receive(:mac?).and_return(true)
+        allow(OS::Mac).to receive(:version).and_return(OS::Mac::Version.new(sierra_os_version))
+      end
+
+      it "doesn't adds a dependency if it doesn't meet OS version requirements" do
+        f = formula 'foo' do
+          url 'foo-1.0'
+
+          uses_from_macos('foo', after: :high_sierra)
+          uses_from_macos('bar', before: :el_capitan)
+        end
+
+        expect(f.class.stable.deps).to be_empty
+        expect(f.class.devel.deps).to be_empty
+        expect(f.class.head.deps).to be_empty
+      end
+
+      it 'allows specifying dependencies after certain version' do
+        f = formula 'foo' do
+          url 'foo-1.0'
+
+          uses_from_macos('foo', after: :el_capitan)
+        end
+
+        expect(f.class.stable.deps.first.name).to eq('foo')
+        expect(f.class.devel.deps.first.name).to eq('foo')
+        expect(f.class.head.deps.first.name).to eq('foo')
+      end
+
+      it 'allows specifying dependencies before certain version' do
+        f = formula 'foo' do
+          url 'foo-1.0'
+
+          uses_from_macos('foo', before: :high_sierra)
+        end
+
+        expect(f.class.stable.deps.first.name).to eq('foo')
+        expect(f.class.devel.deps.first.name).to eq('foo')
+        expect(f.class.head.deps.first.name).to eq('foo')
+      end
+
+      it 'raises an error if passing invalid OS versions' do
+        expect {
+          formula 'foo' do
+            url 'foo-1.0'
+
+            uses_from_macos('foo', after: 'bar', before: :mojave)
+          end
+        }.to raise_error(ArgumentError, 'unknown version "bar"')
+      end
+    end
+  end
 end