encoding.c (enc_set_index): raise instead of rb_bug() for non-encoding capable objects

* Add spec.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64172 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

encoding.c

@@ -819,7 +819,9 @@ rb_enc_get_index(VALUE obj)
 static void
 enc_set_index(VALUE obj, int idx)
 {
-    if (!enc_capable(obj)) rb_bug("enc_set_index: not capable object");
+    if (!enc_capable(obj)) {
+        rb_raise(rb_eArgError, "cannot set encoding on non-encoding capable object");
+    }
 
     if (idx < ENCODING_INLINE_MAX) {
 	ENCODING_SET_INLINED(obj, idx);

spec/ruby/optional/capi/encoding_spec.rb

@@ -30,6 +30,15 @@ describe :rb_enc_set_index, shared: true do
     result = @s.send(@method, str, 1)
     result.first.should == result.last
   end
+
+  ruby_version_is "2.6" do
+    it "raises an ArgumentError for a non-encoding capable object" do
+      obj = Object.new
+      -> {
+        result = @s.send(@method, obj, 1)
+      }.should raise_error(ArgumentError, "cannot set encoding on non-encoding capable object")
+    end
+  end
 end
 
 describe "C-API Encoding function" do