fix and add spec for a string#encode case

Author: Lillian Zhang

spec/ruby/core/string/encode_spec.rb

@@ -19,13 +19,17 @@
     it "returns a copy when Encoding.default_internal is nil" do
       Encoding.default_internal = nil
       str = "あ"
-      str.encode.should_not equal(str)
+      encoded = str.encode
+      encoded.should_not equal(str)
+      encoded.should eql(str)
     end
 
     it "returns a copy for a ASCII-only String when Encoding.default_internal is nil" do
       Encoding.default_internal = nil
       str = "abc"
-      str.encode.should_not equal(str)
+      encoded = str.encode
+      encoded.should_not equal(str)
+      encoded.should eql(str)
     end
 
     it "encodes an ascii substring of a binary string to UTF-8" do
@@ -39,7 +43,9 @@
   describe "when passed to encoding" do
     it "returns a copy when passed the same encoding as the String" do
       str = "あ"
-      str.encode(Encoding::UTF_8).should_not equal(str)
+      encoded = str.encode(Encoding::UTF_8)
+      encoded.should_not equal(str)
+      encoded.should eql(str)
     end
 
     it "round trips a String" do
@@ -75,6 +81,7 @@
       encoded = str.encode("utf-8", "utf-8")
 
       encoded.should_not equal(str)
+      encoded.should eql(str.force_encoding("utf-8"))
       encoded.encoding.should == Encoding::UTF_8
     end
 
@@ -87,14 +94,28 @@
   describe "when passed to, options" do
     it "returns a copy when the destination encoding is the same as the String encoding" do
       str = "あ"
-      str.encode(Encoding::UTF_8, undef: :replace).should_not equal(str)
+      encoded = str.encode(Encoding::UTF_8, undef: :replace)
+      encoded.should_not equal(str)
+      encoded.should eql(str)
     end
   end
 
   describe "when passed to, from, options" do
     it "returns a copy when both encodings are the same" do
       str = "あ"
-      str.encode("utf-8", "utf-8", invalid: :replace).should_not equal(str)
+      encoded = str.encode("utf-8", "utf-8", invalid: :replace)
+      encoded.should_not equal(str)
+      encoded.should eql(str)
+    end
+
+    it "returns a copy in the destination encoding when both encodings are the same" do
+      str = "あ"
+      str.force_encoding("binary")
+      encoded = str.encode("utf-8", "utf-8", invalid: :replace)
+
+      encoded.should_not equal(str)
+      encoded.should eql(str.force_encoding("utf-8"))
+      encoded.encoding.should == Encoding::UTF_8
     end
   end
 end

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

@@ -457,7 +457,7 @@ def encode!(to=undefined, from=undefined, options=undefined)
         status = ec.primitive_convert self.dup, dest, nil, nil, ec.options
         raise ec.last_error unless status == :finished
         return replace(dest)
-      elsif options == 0
+      else
         force_encoding to_enc
       end
     end