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Code like this can be successfully processed by the #[command] proc-macro:

#[command]
pub async fn some_command(ctx: FrameworkContext<Data>, msg: &Message) {}

However, code like this fails:

type Context = FrameworkContext<Data>;

#[command]
pub async fn some_command(ctx: Context, msg: &Message) {}

Error:

error[E0308]: mismatched types
  --> src/crates.rs:42:1
   |
42 | #[command]
   | ^^^^^^^^^^
   | |
   | expected `()`, found struct `Data`
   | expected `serenity_framework::command::Command` because of return type
   |
   = note: expected struct `serenity_framework::command::Command<()>`
              found struct `serenity_framework::command::Command<Data>`

I think the reason for this failure is that #[command] attempts to deduce the user data type by parsing generic parameters of the ctx function argument. Because of the type alias, it can't find any generic parameters and falsely thinks this function uses the default user data (()).

If this suspicion is correct, the issue can be worked around something like this:

// Inside framework crate
pub struct Context<D = DefaultData, E = DefaultError> {
    // ...
}

#[doc(hidden)]
pub trait _ContextTypes {
    type D;
    type E;
}

impl<D, E> _ContextTypes for Context<D, E> {
    type D = D;
    type E = E;
}

The command_attr crate then uses <ARGUMENTTYPE as _ContextTypes>::D and <ARGUMENTTYPE as _ContextTypes>::E instead of trying to deduce the generic parameters from ARGUMENTTYPE.

I have successfully fixed this in my local framework fork. It's based upon #25 so I haven't created a PR yet (that would be a little messy, a PR blocked on a draft PR blocked on a PR in a different repository). Just thought I'd mention to prevent duplicate efforts