We added the policy parameter to all deserializeImpl methods. This allows users to customize deserialization for any specific type, while the remaining types can still use the module-level deserializeImpl method via UFCS.

deserializeImpl is like:

void deserializeImpl(P, T, JT)(ref P policy, ref JT tokenizer, ref T item) if (type constrains) {....}

An example of customized policy is like:

static struct DTStringPolicy {
        void deserializeImpl(JT, T)(ref JT tokenizer, ref T item) {
            static if (is(T == DateTime))
            {
                // Deserialize DateTime from a string
                auto jsonItem = tokenizer.nextSignificant
                    .jsonExpect(JSONToken.String, "Parsing DateTime");
                item = DateTime.fromSimpleString(extractString!string(jsonItem, tokenizer.chain));
            }
            else {
                // default to module behavior
                .deserializeImpl(this, tokenizer, item);
            };
        }
}

https://github.com/schveiguy/jsoniopipe/pull/51
https://github.com/schveiguy/jsoniopipe/pull/52