From 54eed7ee6f8b79b5306842481b680fee15fbd15e Mon Sep 17 00:00:00 2001
From: Kasumi Hanazuki <kasumi@rollingapple.net>
Date: Thu, 1 Oct 2026 16:28:28 +0000
Subject: [PATCH 1/4] IO::Buffer#slice: refine argument and return types

The arguments to IO::Buffer#slice are both optional and
the method returns a subclass instance.

Assisted-by: Codex:gpt-6.1-sol
---
 core/io/buffer.rbs            |  2 +-
 test/stdlib/io/Buffer_test.rb | 28 ++++++++++++++++++++++++++--
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/core/io/buffer.rbs b/core/io/buffer.rbs
index 96326ad7a..259c8255e 100644
--- a/core/io/buffer.rbs
+++ b/core/io/buffer.rbs
@@ -1040,7 +1040,7 @@ class IO
     #     # #<IO::Buffer 0x00007fc3d31e2d80+4 INTERNAL>
     #     # 0x00000000  74 6f 73 74                                     tost
     #
-    def slice: (Integer offset, Integer length) -> Buffer
+    def slice: (?Integer? offset, ?Integer? length) -> self
 
     # <!--
     #   rdoc-file=io_buffer.c
diff --git a/test/stdlib/io/Buffer_test.rb b/test/stdlib/io/Buffer_test.rb
index 72a3f49eb..9d77346fb 100644
--- a/test/stdlib/io/Buffer_test.rb
+++ b/test/stdlib/io/Buffer_test.rb
@@ -411,12 +411,36 @@ def test_size
   end
 
   def test_slice
-    buf = IO::Buffer.new(30)
+    buf = BufferSubclass.new(30)
 
     assert_send_type(
-      "(Integer, Integer) -> IO::Buffer",
+      "() -> IO_Buffer_InstanceTest::BufferSubclass",
+      buf, :slice
+    )
+    assert_send_type(
+      "(Integer) -> IO_Buffer_InstanceTest::BufferSubclass",
+      buf, :slice, 1
+    )
+    assert_send_type(
+      "(Integer, Integer) -> IO_Buffer_InstanceTest::BufferSubclass",
       buf, :slice, 0, 5
     )
+    assert_send_type(
+      "(nil) -> IO_Buffer_InstanceTest::BufferSubclass",
+      buf, :slice, nil
+    )
+    assert_send_type(
+      "(nil, Integer) -> IO_Buffer_InstanceTest::BufferSubclass",
+      buf, :slice, nil, 5
+    )
+    assert_send_type(
+      "(Integer, nil) -> IO_Buffer_InstanceTest::BufferSubclass",
+      buf, :slice, 1, nil
+    )
+    assert_send_type(
+      "(nil, nil) -> IO_Buffer_InstanceTest::BufferSubclass",
+      buf, :slice, nil, nil
+    )
   end
 
   def test_to_s

From dc366b4f15fb4dc56aed241c54f258ef379a77b6 Mon Sep 17 00:00:00 2001
From: Kasumi Hanazuki <kasumi@rollingapple.net>
Date: Thu, 1 Oct 2026 16:30:47 +0000
Subject: [PATCH 2/4] IO::Buffer#transfer: preserves subclass return type

IO::Buffer#transfer returns a subclass instance.

Assisted-by: Codex:gpt-6.1-sol
---
 core/io/buffer.rbs            | 2 +-
 test/stdlib/io/Buffer_test.rb | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/core/io/buffer.rbs b/core/io/buffer.rbs
index 259c8255e..038b61f16 100644
--- a/core/io/buffer.rbs
+++ b/core/io/buffer.rbs
@@ -1073,7 +1073,7 @@ class IO
     #     buffer.null?
     #     # => true
     #
-    def transfer: () -> Buffer
+    def transfer: () -> self
 
     # <!--
     #   rdoc-file=io_buffer.c
diff --git a/test/stdlib/io/Buffer_test.rb b/test/stdlib/io/Buffer_test.rb
index 9d77346fb..2ba0404d4 100644
--- a/test/stdlib/io/Buffer_test.rb
+++ b/test/stdlib/io/Buffer_test.rb
@@ -455,6 +455,10 @@ def test_transfer
       "() -> IO::Buffer",
       IO::Buffer.for(""), :transfer
     )
+    assert_send_type(
+      "() -> IO_Buffer_InstanceTest::BufferSubclass",
+      BufferSubclass.new(4), :transfer
+    )
   end
 
   def test_valid?

From 6c98c6b07b6d1d5748c78137855b01a5f8ec86fb Mon Sep 17 00:00:00 2001
From: Kasumi Hanazuki <kasumi@rollingapple.net>
Date: Thu, 1 Oct 2026 16:33:24 +0000
Subject: [PATCH 3/4] IO::Buffer.for: preserve subclass types

Assisted-by: Codex:gpt-6.1-sol
---
 core/io/buffer.rbs            |  4 ++--
 test/stdlib/io/Buffer_test.rb | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/core/io/buffer.rbs b/core/io/buffer.rbs
index 038b61f16..8b8e413c5 100644
--- a/core/io/buffer.rbs
+++ b/core/io/buffer.rbs
@@ -124,8 +124,8 @@ class IO
     #       # => "Test"
     #     end
     #
-    def self.for: (string) -> Buffer
-                | [A] (string) { (Buffer) -> A } -> A
+    def self.for: (string) -> instance
+                | [A] (string) { (instance) -> A } -> A
 
     # <!--
     #   rdoc-file=io_buffer.c
diff --git a/test/stdlib/io/Buffer_test.rb b/test/stdlib/io/Buffer_test.rb
index 2ba0404d4..764925332 100644
--- a/test/stdlib/io/Buffer_test.rb
+++ b/test/stdlib/io/Buffer_test.rb
@@ -20,6 +20,21 @@ def test_for
         "(string) { (IO::Buffer) -> String } -> String",
         IO::Buffer, :for, source
       ) { "result" }
+
+      assert_send_type(
+        "(string) -> IO_Buffer_InstanceTest::BufferSubclass",
+        IO_Buffer_InstanceTest::BufferSubclass, :for, source
+      )
+
+      assert_send_type(
+        "(string) { (IO_Buffer_InstanceTest::BufferSubclass) -> Integer } -> Integer",
+        IO_Buffer_InstanceTest::BufferSubclass, :for, source
+      ) { 42 }
+
+      assert_send_type(
+        "(string) { (IO_Buffer_InstanceTest::BufferSubclass) -> String } -> String",
+        IO_Buffer_InstanceTest::BufferSubclass, :for, source
+      ) { "result" }
     end
   end
 

From acd1e7112beee78dc40827adbc44d3fbe3031a16 Mon Sep 17 00:00:00 2001
From: Kasumi Hanazuki <kasumi@rollingapple.net>
Date: Thu, 1 Oct 2026 16:33:37 +0000
Subject: [PATCH 4/4] IO::Buffer.string: yield subclass instances

Assisted-by: Codex:gpt-6.1-sol
---
 core/io/buffer.rbs            | 2 +-
 test/stdlib/io/Buffer_test.rb | 9 +++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/core/io/buffer.rbs b/core/io/buffer.rbs
index 8b8e413c5..285f16a50 100644
--- a/core/io/buffer.rbs
+++ b/core/io/buffer.rbs
@@ -200,7 +200,7 @@ class IO
     #     end
     #     # => "Ruby"
     #
-    def self.string: (int) { (Buffer) -> void } -> String
+    def self.string: (int) { (instance) -> void } -> String
 
     # <!--
     #   rdoc-file=io_buffer.c
diff --git a/test/stdlib/io/Buffer_test.rb b/test/stdlib/io/Buffer_test.rb
index 764925332..67ff0c242 100644
--- a/test/stdlib/io/Buffer_test.rb
+++ b/test/stdlib/io/Buffer_test.rb
@@ -77,8 +77,13 @@ def test_string
     with_int 10 do |int|
       assert_send_type(
         "(int) { (IO::Buffer) -> nil } -> String",
-        IO::Buffer, :string, int, &proc { nil }
-      )
+        IO::Buffer, :string, int
+      ) { nil }
+
+      assert_send_type(
+        "(int) { (IO_Buffer_InstanceTest::BufferSubclass) -> nil } -> String",
+        IO_Buffer_InstanceTest::BufferSubclass, :string, int
+      ) { nil }
     end
   end
 end