Refine types for IO::Buffer methods that return subclass instances

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
@@ -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
@@ -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
@@ -1073,7 +1073,7 @@ class IO
     #     buffer.null?
     #     # => true
     #
-    def transfer: () -> Buffer
+    def transfer: () -> self
 
     # <!--
     #   rdoc-file=io_buffer.c

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
 
@@ -62,8 +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
@@ -411,12 +431,36 @@ 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
@@ -431,6 +475,10 @@ def test_transfer
       "() -> IO::Buffer",
       IO::Buffer.for(""), :transfer
     )
+    assert_send_type(
+      "() -> IO_Buffer_InstanceTest::BufferSubclass",
+      BufferSubclass.new(4), :transfer
+    )
   end
 
   def test_valid?