Invalid generic declaration when using FRAME macro pallet::error

Question

I want to add custom error types to a working pallet (permalink duniter-wot/src/lib.rs). In this current state, cargo check pass successfully.

According to Substrate documentation frame-macros/#palleterror, I'm adding inside the #[frame_support::pallet] block a #[pallet::error] macro with an enumeration named Error with a generic type like:

#[pallet::error]
pub enum Error<T> {
    /// variant one
    VariantOne,
    /// variant two
    VariantTwo,
}

But as soon as I do this, cargo check returns a long (> 700 lines) error trace in which the first element is:

error: Invalid generic declaration, trait is defined with instance but generic use none
  --> pallets/duniter-wot/src/lib.rs:99:20
   |
99 |     pub enum Error<T> {
   |                    ^

The question, more than "how to fix this" would be "how am I supposed to understand this hint" and "what resources/tools/documentation could help me debug myself".

Answer 1

It looks like your pallet is instantiatable. This requires you to additionally pass in the instance type like such. In the easiest case it looks like this:

#[pallet::error]
pub enum Error<T, I = ()> {

Making a pallet instantiatable is useful when you plan on having more than one instance deployed within the same runtime.

what resources/tools/documentation could help me debug myself

For me personally it is enough to read examples from the Substrate repo. For example pallet-referenda.