presto-main/src/main/java/io/prestosql/metadata/TypeRegistry.java/TypeRegistry/addType
class TypeRegistry:    public void addType(String alias, Type type)
    {
        requireNonNull(alias, "alias is null");
        requireNonNull(type, "type is null");

        Type existingType = types.putIfAbsent(new TypeSignature(alias), type);
        checkState(existingType == null || existingType.equals(type), "Alias %s is already mapped to %s", alias, type);
    }