E0049: Linear Method Call Violation

Only one method call is allowed on a narrowed ABI variable per if ... is block. Calling a second method is a hard error because ABI method calls can change the Utxo's state, potentially invalidating the ABI type.

Example (error)

abi MyAbi {
    fn transfer(amount: i64) -> bool;
    fn balance() -> i64;
}

fn test(x: Utxo) {
    if x is MyAbi {
        x.transfer(100);  // OK: first method call
        x.balance();       // Error: second method call on narrowed variable
    }
}

Fix

Use separate if ... is blocks for each method call:

fn test(x: Utxo) {
    if x is MyAbi {
        x.transfer(100);
    }
    if x is MyAbi {
        x.balance();
    }
}