Can't reference fields from PROTO javascript functions if not referenced directly

[brettle]

As a test case, consider:

#VRML_SIM R2023b utf8
# template language: javascript

PROTO ExampleProto [
    field SFFloat field1 1
    field SFFloat field2 5
]
{
  Solid {
%<
function sum(fieldsObj) {
  return fieldsObj.field1.value + fieldsObj.field2.value;
}
>%
       radarCrossSection %<= sum(fields) >%
  }
}

When I insert that proto, I get:

ERROR: '.../ExampleProto.proto': error: Template engine error: failed to execute JavaScript template: Cannot read property 'value' of undefined.

I suspect I'm running up against this:

webots/src/webots/vrml/WbProtoModel.cpp

Lines 277 to 295 in 69681a9

	foreach (WbFieldModel *model, mFieldModels) {
	// regex test cases:
	// "You know nothing, John Snow." => false
	// "%{=fields.model->name()}%" => false
	// "%{= fields.model->name().value.x }% %{= fields.model->name().value.y }%" => true
	// "abc %{= fields.model->name().value.y }% def" => true
	// "%{= 17 % fields.model->name().value.y * 88 }%" => true
	// "fields.model->name().value.y" => false
	// "%{}% fields.model->name().value.y %{}%" => false
	// "%{ a = \"fields.model->name().value.y\" }%" => false
	// "%{= \"fields.model->name().value.y\" }%" => false
	// "%{= fields.model->name().value.y }%" => true
	if (token->word().contains(QRegularExpression(QString("%1(?:(?!%2|\").)*fields\\.%3(?:(?!%4|\").)*%5")
	.arg(open)
	.arg(close)
	.arg(QRegularExpression::escape(model->name()))
	.arg(close)
	.arg(close))))
	model->setTemplateRegenerator(true);