presto-spi/src/main/java/io/prestosql/spi/type/TypeSignature.java/TypeSignature/getTypeParametersAsTypeSignatures
class TypeSignature:    public List<TypeSignature> getTypeParametersAsTypeSignatures()
    {
        List<TypeSignature> result = new ArrayList<>();
        for (TypeSignatureParameter parameter : parameters) {
            if (parameter.getKind() != ParameterKind.TYPE) {
                throw new IllegalStateException(
                        format("Expected all parameters to be TypeSignatures but [%s] was found", parameter.toString()));
            }
            result.add(parameter.getTypeSignature());
        }
        return result;
    }