Add option for a truthy? method on drops for use in if/unless statements.

lib/liquid/tags/if.rb

@@ -56,7 +56,7 @@ def render_to_output_buffer(context, output)
           block.evaluate(context),
         )
 
-        if result
+        if result.respond_to?(:truthy?) ? result.truthy? : result
           return block.attachment.render_to_output_buffer(context, output)
         end
       end

lib/liquid/tags/unless.rb

@@ -26,7 +26,7 @@ def render_to_output_buffer(context, output)
         first_block.evaluate(context),
       )
 
-      unless result
+      unless result.respond_to?(:truthy?) ? result.truthy? : result
         return first_block.attachment.render_to_output_buffer(context, output)
       end
 
@@ -36,7 +36,7 @@ def render_to_output_buffer(context, output)
           block.evaluate(context),
         )
 
-        if result
+        if result.respond_to?(:truthy?) ? result.truthy? : result
           return block.attachment.render_to_output_buffer(context, output)
         end
       end

test/unit/tags/if_tag_unit_test.rb

@@ -7,4 +7,13 @@ def test_if_nodelist
     template = Liquid::Template.parse('{% if true %}IF{% else %}ELSE{% endif %}')
     assert_equal(['IF', 'ELSE'], template.root.nodelist[0].nodelist.map(&:nodelist).flatten)
   end
+
+  def test_support_truthy
+    falsey = Class.new(Liquid::Drop) { def truthy? = false }.new
+    truthy = Class.new(Liquid::Drop) { def truthy? = true }.new
+    template = '{% if obj %}IF{% else %}ELSE{% endif %}'
+    assert_template_result('ELSE', template, { 'obj' => falsey })
+    assert_template_result('IF', template, { 'obj' => truthy })
+    assert_template_result('IF', template, { 'obj' => "foo" }) # truthy? not defined
+  end
 end

test/unit/tags/unless_tag_test.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class IfTagUnitTest < Minitest::Test
+  def test_support_truthy
+    falsey = Class.new(Liquid::Drop) { def truthy? = false }.new
+    truthy = Class.new(Liquid::Drop) { def truthy? = true }.new
+    template = '{% unless obj %}IF{% else %}ELSE{% endunless %}'
+    assert_template_result('ELSE', template, { 'obj' => truthy })
+    assert_template_result('IF', template, { 'obj' => falsey })
+    assert_template_result('ELSE', template, { 'obj' => "truthy? not defined" })
+  end
+end