From 65a1c167b33a8d8fe28ec657b6108c7f35258873 Mon Sep 17 00:00:00 2001
From: Guilherme Carreiro <guilherme.carreiro@shopify.com>
Date: Thu, 2 Oct 2025 12:51:42 +0200
Subject: [PATCH] Add rigid_parse to `case/when`

---
 lib/liquid/parser_switching.rb       |   4 +
 lib/liquid/tags/case.rb              |  49 +++++++++--
 test/unit/tags/case_tag_unit_test.rb | 120 +++++++++++++++++++++++++++
 3 files changed, 167 insertions(+), 6 deletions(-)

diff --git a/lib/liquid/parser_switching.rb b/lib/liquid/parser_switching.rb
index 78b86b23..799250a0 100644
--- a/lib/liquid/parser_switching.rb
+++ b/lib/liquid/parser_switching.rb
@@ -37,6 +37,10 @@ module Liquid
       end
     end
 
+    def rigid_mode?
+      parse_context.error_mode == :rigid
+    end
+
     private
 
     def rigid_parse_with_error_context(markup)
diff --git a/lib/liquid/tags/case.rb b/lib/liquid/tags/case.rb
index 6b67601f..bdf329a2 100644
--- a/lib/liquid/tags/case.rb
+++ b/lib/liquid/tags/case.rb
@@ -31,12 +31,7 @@ module Liquid
     def initialize(tag_name, markup, options)
       super
       @blocks = []
-
-      if markup =~ Syntax
-        @left = parse_expression(Regexp.last_match(1))
-      else
-        raise SyntaxError, options[:locale].t("errors.syntax.case")
-      end
+      parse_with_selected_parser(markup)
     end
 
     def parse(tokens)
@@ -91,9 +86,51 @@ module Liquid
 
     private
 
+    def rigid_parse(markup)
+      parser = @parse_context.new_parser(markup)
+      @left = safe_parse_expression(parser)
+      parser.consume(:end_of_string)
+    end
+
+    def strict_parse(markup)
+      lax_parse(markup)
+    end
+
+    def lax_parse(markup)
+      if markup =~ Syntax
+        @left = parse_expression(Regexp.last_match(1))
+      else
+        raise SyntaxError, options[:locale].t("errors.syntax.case")
+      end
+    end
+
     def record_when_condition(markup)
       body = new_body
 
+      if rigid_mode?
+        parse_rigid_when(markup, body)
+      else
+        parse_lax_when(markup, body)
+      end
+    end
+
+    def parse_rigid_when(markup, body)
+      parser = @parse_context.new_parser(markup)
+
+      loop do
+        expr = safe_parse_expression(parser)
+        block = Condition.new(@left, '==', expr)
+        block.attach(body)
+        @blocks << block
+
+        # Temporarily until support :or lexeme.
+        break unless parser.id?('or') || parser.consume?(:comma)
+      end
+
+      parser.consume(:end_of_string)
+    end
+
+    def parse_lax_when(markup, body)
       while markup
         unless markup =~ WhenSyntax
           raise SyntaxError, options[:locale].t("errors.syntax.case_invalid_when")
diff --git a/test/unit/tags/case_tag_unit_test.rb b/test/unit/tags/case_tag_unit_test.rb
index a94d167f..687cf9d9 100644
--- a/test/unit/tags/case_tag_unit_test.rb
+++ b/test/unit/tags/case_tag_unit_test.rb
@@ -9,4 +9,124 @@ class CaseTagUnitTest < Minitest::Test
     template = Liquid::Template.parse('{% case var %}{% when true %}WHEN{% else %}ELSE{% endcase %}')
     assert_equal(['WHEN', 'ELSE'], template.root.nodelist[0].nodelist.map(&:nodelist).flatten)
   end
+
+  def test_case_with_trailing_element
+    template = <<~LIQUID
+      {%- case 1 bar -%}
+        {%- when 1 -%}
+          one
+        {%- else -%}
+          two
+      {%- endcase -%}
+    LIQUID
+
+    [:lax, :strict].each do |mode|
+      with_error_mode(mode) { assert_template_result("one", template) }
+    end
+
+    with_error_mode(:rigid) do
+      error = assert_raises(Liquid::SyntaxError) { Template.parse(template) }
+
+      assert_match(/Expected end_of_string but found/, error.message)
+    end
+  end
+
+  def test_case_when_trailing_element
+    template = <<~LIQUID
+      {%- case 1 -%}
+        {%- when 1 bar -%}
+          one
+        {%- else -%}
+          two
+      {%- endcase -%}
+    LIQUID
+
+    [:lax, :strict].each do |mode|
+      with_error_mode(mode) { assert_template_result("one", template) }
+    end
+
+    with_error_mode(:rigid) do
+      error = assert_raises(Liquid::SyntaxError) { Template.parse(template) }
+
+      assert_match(/Expected end_of_string but found/, error.message)
+    end
+  end
+
+  def test_case_when_with_comma
+    template = <<~LIQUID
+      {%- case 1 -%}
+        {%- when 2, 1 -%}
+          one
+        {%- else -%}
+          two
+      {%- endcase -%}
+    LIQUID
+
+    [:lax, :strict, :rigid].each do |mode|
+      with_error_mode(mode) { assert_template_result("one", template) }
+    end
+  end
+
+  def test_case_when_with_or
+    template = <<~LIQUID
+      {%- case 1 -%}
+        {%- when 2 or 1 -%}
+          one
+        {%- else -%}
+          two
+      {%- endcase -%}
+    LIQUID
+
+    [:lax, :strict, :rigid].each do |mode|
+      with_error_mode(mode) { assert_template_result("one", template) }
+    end
+  end
+
+  def test_case_with_invalid_expression
+    template = <<~LIQUID
+      {%- case foo=>bar -%}
+        {%- when 'baz' -%}
+          one
+        {%- else -%}
+          two
+      {%- endcase -%}
+    LIQUID
+    assigns = { 'foo' => { 'bar' => 'baz' } }
+
+    [:lax, :strict].each do |mode|
+      with_error_mode(mode) do
+        assert_template_result("one", template, assigns)
+      end
+    end
+
+    with_error_mode(:rigid) do
+      error = assert_raises(Liquid::SyntaxError) { Template.parse(template) }
+
+      assert_match(/Unexpected character =/, error.message)
+    end
+  end
+
+  def test_case_when_with_invalid_expression
+    template = <<~LIQUID
+      {%- case 'baz' -%}
+        {%- when foo=>bar -%}
+          one
+        {%- else -%}
+          two
+      {%- endcase -%}
+    LIQUID
+    assigns = { 'foo' => { 'bar' => 'baz' } }
+
+    [:lax, :strict].each do |mode|
+      with_error_mode(mode) do
+        assert_template_result("one", template, assigns)
+      end
+    end
+
+    with_error_mode(:rigid) do
+      error = assert_raises(Liquid::SyntaxError) { Template.parse(template) }
+
+      assert_match(/Unexpected character =/, error.message)
+    end
+  end
 end