V3 macro style pallet failed to compile against latest polkadot release

Question

Trying to compile a V3 macro style pallet against the latest polkadot (0.9.24) dependencies fails with error:

error[E0277]: the trait bound `<T as frame_system::Config>::AccountId: std::default::Default` is not satisfied
   --> pallet-tft-bridge/src/lib.rs:138:1
    |
138 | / decl_storage! {
139 | |     trait Store for Module<T: Config> as TFTBridgeModule {
140 | |         pub Validators get(fn validator_accounts): Vec<T::AccountId>;
141 | |         pub FeeAccount get(fn fee_account): T::AccountId;
...   |
180 | |     }
181 | | }
    | |_^ the trait `std::default::Default` is not implemented for `<T as frame_system::Config>::AccountId`
    |
    = note: this error originates in the macro `decl_storage` (in Nightly builds, run with -Z macro-backtrace for more info)

Do I need to rewrite my pallet using the new syntax style in 4.0.0-dev?

Answer 1

In this PR the Default bound on AccountId was removed.

As Ron mentioned, the answer to this is to update your code to:

decl_storage! {
    trait Store for Module<T: Config> as TFTBridgeModule {
        pub Validators get(fn validator_accounts): Vec<T::AccountId>;
        pub FeeAccount get(fn fee_account): Option<T::AccountId>;
    }
}

Where FeeAccount returns an Option rather than directly the T::AccountId type.

Another option is to use the syntax:

decl_storage! {
    trait Store for Module<T: Config> as TFTBridgeModule {
        pub Validators get(fn validator_accounts): Vec<T::AccountId>;
        pub FeeAccount get(fn fee_account): T::AccountId = SomeDefaultAccount;
    }
}

Where you can specify what the default value for this storage should be, which allows you to return a value without an option, but also not depend on the Default trait.