Make IO#set_encoding with binary external encoding use nil internal encoding

This was already the behavior when a single `'external:internal'`
encoding specifier string was passed.  This makes the behavior
consistent for the case where separate external and internal
encoding specifiers are provided.

While here, fix the IO#set_encoding method documentation to
state that either the first or second argument can be a string
with an encoding name, and describe the behavior when the
external encoding is binary.

Fixes [Bug #18899]

1 files changed, 18 insertions, 6 deletions

diff --git a/spec/ruby/core/io/gets_spec.rb b/spec/ruby/core/io/gets_spec.rb
index 07be99f400..2d5e3d1ae3 100644
--- a/spec/ruby/core/io/gets_spec.rb
+++ b/spec/ruby/core/io/gets_spec.rb
@@ -301,11 +301,23 @@ describe "IO#gets" do
     @io.gets.encoding.should == Encoding::BINARY
   end
 
-  it "transcodes to internal encoding if the IO object's external encoding is BINARY" do
-    Encoding.default_external = Encoding::BINARY
-    Encoding.default_internal = Encoding::UTF_8
-    @io = new_io @name, 'r'
-    @io.set_encoding Encoding::BINARY, Encoding::UTF_8
-    @io.gets.encoding.should == Encoding::UTF_8
+  ruby_version_is ''...'3.3' do
+    it "transcodes to internal encoding if the IO object's external encoding is BINARY" do
+      Encoding.default_external = Encoding::BINARY
+      Encoding.default_internal = Encoding::UTF_8
+      @io = new_io @name, 'r'
+      @io.set_encoding Encoding::BINARY, Encoding::UTF_8
+      @io.gets.encoding.should == Encoding::UTF_8
+    end
+  end
+
+  ruby_version_is '3.3' do
+    it "ignores the internal encoding if the IO object's external encoding is BINARY" do
+      Encoding.default_external = Encoding::BINARY
+      Encoding.default_internal = Encoding::UTF_8
+      @io = new_io @name, 'r'
+      @io.set_encoding Encoding::BINARY, Encoding::UTF_8
+      @io.gets.encoding.should == Encoding::BINARY
+    end
   end
 end