presto-main/src/main/java/io/prestosql/operator/scalar/annotations/OperatorValidator.java/OperatorValidator/formatSignature
class OperatorValidator:    private static String formatSignature(OperatorType operatorType, TypeSignature returnType, List<TypeSignature> argumentTypes)
    {
        return operatorType + "(" + Joiner.on(", ").join(argumentTypes) + ")::" + returnType;
    }