presto-spi/src/main/java/io/prestosql/spi/type/NamedTypeSignature.java/NamedTypeSignature/NamedTypeSignature
class NamedTypeSignature:    public NamedTypeSignature(Optional<RowFieldName> fieldName, TypeSignature typeSignature)
    {
        this.fieldName = requireNonNull(fieldName, "fieldName is null");
        this.typeSignature = requireNonNull(typeSignature, "typeSignature is null");
    }