[GR-18163] Fix IO#set_encoding

PullRequest: truffleruby/3534

spec/ruby/core/io/set_encoding_spec.rb

@@ -1,14 +1,27 @@
 require_relative '../../spec_helper'
 
 describe :io_set_encoding_write, shared: true do
-  it "sets the encodings to nil" do
+  it "sets the encodings to nil when they were set previously" do
     @io = new_io @name, "#{@object}:ibm437:ibm866"
     @io.set_encoding nil, nil
 
     @io.external_encoding.should be_nil
     @io.internal_encoding.should be_nil
   end
 
+  it "sets the encodings to nil when the IO is built with no explicit encoding" do
+    @io = new_io @name, @object
+
+    # Checking our assumptions first
+    @io.external_encoding.should be_nil
+    @io.internal_encoding.should be_nil
+
+    @io.set_encoding nil, nil
+
+    @io.external_encoding.should be_nil
+    @io.internal_encoding.should be_nil
+  end
+
   it "prevents the encodings from changing when Encoding defaults are changed" do
     @io = new_io @name, "#{@object}:utf-8:us-ascii"
     @io.set_encoding nil, nil
@@ -38,6 +51,7 @@
     @external = Encoding.default_external
     @internal = Encoding.default_internal
 
+    # The defaults
     Encoding.default_external = Encoding::UTF_8
     Encoding.default_internal = nil
 
@@ -113,6 +127,22 @@
   describe "with 'a+' mode" do
     it_behaves_like :io_set_encoding_write, nil, "a+"
   end
+
+  describe "with standard IOs" do
+    it "correctly resets them" do
+      STDOUT.external_encoding.should == nil
+      STDOUT.internal_encoding.should == nil
+
+      begin
+        STDOUT.set_encoding(Encoding::US_ASCII, Encoding::ISO_8859_1)
+      ensure
+        STDOUT.set_encoding(nil, nil)
+      end
+
+      STDOUT.external_encoding.should == nil
+      STDOUT.internal_encoding.should == nil
+    end
+  end
 end
 
 describe "IO#set_encoding" do

src/main/ruby/truffleruby/core/argf.rb

@@ -83,7 +83,7 @@ def initialize(argv = ARGV, *others)
     #
     def binmode
       @binmode = true
-      set_encoding(Encoding::ASCII_8BIT)
+      set_encoding(Encoding::BINARY)
       self
     end