Check encoding compatibility in String#casecmp

spec/core/string/casecmp_spec.rb

@@ -30,9 +30,7 @@
   end
 
   it "returns nil if incompatible encodings" do
-    NATFIXME 'returns nil if incompatible encodings', exception: SpecFailedException do
-      "あれ".casecmp("れ".encode(Encoding::EUC_JP)).should be_nil
-    end
+    "あれ".casecmp("れ".encode(Encoding::EUC_JP)).should be_nil
   end
 
   describe "in UTF-8 mode" do
@@ -145,9 +143,7 @@
   end
 
   it "returns nil if incompatible encodings" do
-    NATFIXME 'returns nil if incompatible encodings', exception: SpecFailedException do
-      "あれ".casecmp?("れ".encode(Encoding::EUC_JP)).should be_nil
-    end
+    "あれ".casecmp?("れ".encode(Encoding::EUC_JP)).should be_nil
   end
 
   describe 'for UNICODE characters' do

src/string_object.cpp

@@ -2801,19 +2801,38 @@ Value StringObject::casecmp(Env *env, Value other) {
     other = StringObject::try_convert(env, other);
     if (other->is_nil())
         return NilObject::the();
+
+    auto other_str = other->as_string_or_raise(env);
+
+    if (is_empty() && other_str->is_empty())
+        return Value::integer(0);
+
+    if (!negotiate_compatible_encoding(other_str))
+        return NilObject::the();
+
     auto str1 = this->downcase(env, "ascii"_s, nullptr);
-    auto str2 = other->as_string()->downcase(env, "ascii"_s, nullptr);
+    auto str2 = other_str->downcase(env, "ascii"_s, nullptr);
     return str1->cmp(env, Value(str2));
 }
 
 Value StringObject::is_casecmp(Env *env, Value other) {
     other = StringObject::try_convert(env, other);
     if (other->is_nil())
         return NilObject::the();
+
+    auto other_str = other->as_string_or_raise(env);
+
+    if (is_empty() && other_str->is_empty())
+        return Value::integer(0);
+
+    if (!negotiate_compatible_encoding(other_str))
+        return NilObject::the();
+
     auto str1 = this->downcase(env, "fold"_s, nullptr);
-    auto str2 = other->as_string()->downcase(env, "fold"_s, nullptr);
+    auto str2 = other_str->downcase(env, "fold"_s, nullptr);
     if (str1->string() == str2->string())
         return TrueObject::the();
+
     return FalseObject::the();
 }