swc-project/swc · error

generic parameter other than type

Error message

generic parameter other than type

What it means

`quote!` from swc_ecma_quote_macros reads the type written after `as` (e.g. `quote!("a + b" as Box<Expr>)`) to decide how to parse and wrap the fragment. `extract_generic` only accepts an angle-bracketed argument list whose first argument is a plain type; any other generic-argument kind (lifetime or const argument) hits `unimplemented!("generic parameter other than type")` at compile time.

Source

Thrown at crates/swc_ecma_quote_macros/src/ret_type.rs:91

    let mut parser = Parser::new_from(lexer);
    op(&mut parser)
        .map_err(|err| anyhow!("{err:?}"))
        .with_context(|| format!("failed to parse input as `{}`", type_name::<T>()))
        .map(|val| BoxWrapper(Box::new(val)))
}

fn extract_generic<'a>(name: &str, ty: &'a Type) -> Option<&'a Type> {
    if let Type::Path(p) = ty {
        let last = p.path.segments.last().unwrap();

        if !last.arguments.is_empty() && last.ident == name {
            match &last.arguments {
                PathArguments::AngleBracketed(tps) => {
                    let arg = tps.args.first().unwrap();

                    match arg {
                        GenericArgument::Type(arg) => return Some(arg),
                        _ => unimplemented!("generic parameter other than type"),
                    }
                }
                _ => unimplemented!("Box() -> T or Box without a type parameter"),
            }
        }
    }

    None
}

View on GitHub (pinned to d7d7434666)

Solutions

1. Use the supported forms: `as Box<Expr>` or `as Option<Expr>` with a plain type argument
2. Wrap a parameterized inner type in a type alias so the outer argument stays a plain type

Example fix

// before
let node = quote!("self.field" as Box<'a>);

// after
let node = quote!("self.field" as Box<Expr>);

When it happens

Trigger: Write a quote invocation whose wrapper type's first generic argument is not a type — e.g. `Box<'a>` or a const-generic parameter — instead of the supported `Box<T>` / `Option<T>` shapes.

Common situations: Writing custom visitor or matcher macros on top of swc's quote macros; refactors that move lifetimes into the wrapper position; meta-macros generating quote calls with spliced tokens.

Related errors

• Box() -> T or Box without a type parameter
• Fast-path injection for Fold / VisitMut where pattern is not
• Unknown visitor type: {:?}
• union
• {err:?}

AI-assisted analysis of swc-project/swc@d7d7434666 (2026-08-16). Data as JSON: /api/errors/7485127efb4eed85. Report an issue: GitHub.