swc-project/swc · error

multiple constructor?

Error message

multiple constructor?

What it means

The decorator transform (swc_ecma_transforms_proposal::decorators) builds property descriptors from a class body; by the time this filter_map runs, the constructor has already been extracted and removed from the member list, so encountering another ClassMember::Constructor is treated as `multiple constructor?`. The parser itself rejects duplicate constructors, so this unreachable! signals a mutated/hand-built class AST rather than parsed source.

Source

Thrown at crates/swc_ecma_transforms_proposal/src/decorators/mod.rs:439

                                    }
                                    .into(),
                                ),
                            },
                        )))))
                        .collect(),
                    }
                    .as_arg(),
                )
            }};
        }

        let descriptors = class
            .body
            .into_iter()
            .filter_map(|member| {
                //
                match member {
                    ClassMember::Constructor(_) => unreachable!("multiple constructor?"),
                    ClassMember::Empty(_) | ClassMember::TsIndexSignature(_) => None,
                    ClassMember::Method(method) => {
                        let fn_name = match method.key {
                            PropName::Ident(ref i) => Some(i.clone()),
                            PropName::Str(ref s) => s
                                .value
                                .as_str()
                                .map(|sym| IdentName::new(Atom::from(sym), s.span)),
                            _ => None,
                        };
                        let key_prop_value = Box::new(prop_name_to_expr_value(method.key.clone()));

                        fold_method!(method, fn_name, key_prop_value)
                    }
                    ClassMember::PrivateMethod(method) => {
                        let fn_name = Ident::new_no_ctxt(
                            format!("_{}", method.key.name).into(),
                            method.key.span,

View on GitHub (pinned to 5176682b65)

Solutions

1. Validate the class body before the decorator pass: exactly zero-or-one Constructor member.
2. Fix the upstream pass that injects or duplicates constructor members into the class body.
3. If the class comes from parsed source, report an swc bug with the repro (the parser should have rejected it).

Example fix

// before: synthesized AST accidentally carries two constructors
class.body.push(ClassMember::Constructor(second_ctor));

// after: mutate in place, keep a single constructor
if let Some(ClassMember::Constructor(existing)) = class.body.iter_mut().find(|m| matches!(m, ClassMember::Constructor(_))) {
    *existing = new_ctor;
}

use swc_ecma_ast::{ClassMember, Class};
fn single_constructor(c: &Class) -> bool {
    c.body.iter().filter(|m| matches!(m, ClassMember::Constructor(_))).count() <= 1
}
// assert!(single_constructor(&class)) before the decorators pass

fn has_valid_ctor_count(c: &Class) -> bool { c.body.iter().filter(|m| matches!(m, ClassMember::Constructor(_))).count() <= 1 }

When it happens

Trigger: Programmatically constructed or transform-modified Class AST where two ClassMember::Constructor nodes exist (custom decorators, class-mutation passes, AST round-trips through FFI/serde that duplicate members), compiled with the 2022-era decorators option.

Common situations: Decorator plugin ecosystems that rewrite class bodies before the decorator pass; codemods synthesizing classes; swc AST serialized by one version and deserialized by another re-adding members.

Related errors

• unknown compound assignment operator
• getter/setter property be compiled as CompiledProp::Accessor
• Unknown bit operator {:?}
• unknown binary operator: {:?}
• Invalid attempt to iterate non-iterable instance. In order t

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/9914c4f291cc07d6. Report an issue: GitHub.