From d225bb8b464e4e03d2eb6c09ef15adf727af9e2b Mon Sep 17 00:00:00 2001
From: Jeff Zhang <jeff.j.zhang@shopify.com>
Date: Tue, 20 Jan 2026 10:50:43 -0500
Subject: ZJIT: Compile IsA into load + compare for String/Array/Hash (#15878)

Resolves https://github.com/Shopify/ruby/issues/880

Implemented this by using the code generation for `GuardType` as a reference.
Not sure if this is the best way to go about it, but it seems to work.
---
 test/ruby/test_zjit.rb | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

(limited to 'test/ruby')

diff --git a/test/ruby/test_zjit.rb b/test/ruby/test_zjit.rb
index ad2df806d5..2cfe9dd7e3 100644
--- a/test/ruby/test_zjit.rb
+++ b/test/ruby/test_zjit.rb
@@ -4417,6 +4417,60 @@ class TestZJIT < Test::Unit::TestCase
     }, call_threshold: 14, num_profiles: 5
   end
 
+  def test_is_a_string_special_case
+    assert_compiles '[true, false, false, false, false, false]', %q{
+      def test(x)
+        x.is_a?(String)
+      end
+      test("foo")
+      [test("bar"), test(1), test(false), test(:foo), test([]), test({})]
+    }
+  end
+
+  def test_is_a_array_special_case
+    assert_compiles '[true, true, false, false, false, false, false]', %q{
+      def test(x)
+        x.is_a?(Array)
+      end
+      test([])
+      [test([1,2,3]), test([]), test(1), test(false), test(:foo), test("foo"), test({})]
+    }
+  end
+
+  def test_is_a_hash_special_case
+    assert_compiles '[true, true, false, false, false, false, false]', %q{
+      def test(x)
+        x.is_a?(Hash)
+      end
+      test({})
+      [test({:a => "b"}), test({}), test(1), test(false), test(:foo), test([]), test("foo")]
+    }
+  end
+
+  def test_is_a_hash_subclass
+    assert_compiles 'true', %q{
+      class MyHash < Hash
+      end
+      def test(x)
+        x.is_a?(Hash)
+      end
+      test({})
+      test(MyHash.new)
+    }
+  end
+
+  def test_is_a_normal_case
+    assert_compiles '[true, false]', %q{
+      class MyClass
+      end
+      def test(x)
+        x.is_a?(MyClass)
+      end
+      test("a")
+      [test(MyClass.new), test("a")]
+    }
+  end
+
   private
 
   # Assert that every method call in `test_script` can be compiled by ZJIT
-- 
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