swc-project/swc · error

ClassMember

Error message

ClassMember::{:?}

What it means

In the non-legacy (2022 proposal) decorator transform, SWC builds per-member descriptor info only for constructors, methods, and class properties (ClassMember::Constructor/Method/ClassProp). Any other class member — static blocks, TS index signatures, private properties with decorators, auto-accessors, empty members — falls into the catch-all and panics with `unimplemented!("ClassMember::...")`.

Solutions

1. Remove the unsupported member from the decorated class: move static-block init into a static field or the constructor, drop the index signature, or undecorate private fields
2. Switch to "legacy": true decorators, which use a different code path that tolerates these members
3. Upgrade swc_ecma_transforms_proposal in case the member kind is now handled

Example fix

// before
@decorator
class C {
    static { register(C); }
    [k: string]: any;
}

// after
@decorator
class C {
    static _init = register(C);
}

// TS-side precheck of source about to be compiled with new decorators
const UNSUPPORTED = /static\s*\{|\[\s*\w+\s*:\s*\w+\s*\]|@\w+\s+#\w+/;
if (decoratorsEnabled && !legacy && UNSUPPORTED.test(src)) {
  throw new Error('Decorated class contains static block / index signature / decorated private member');
}

When it happens

Trigger: Compiling with jsc.decorators (legacy false) a class that contains a `static { ... }` block, a TS index signature like `[k: string]: any`, a decorated `#privateField`, or another unsupported member while decorators are enabled for that class.

Common situations: Modern TS codebases mixing decorators with static initialization blocks or index signatures; libraries like TypeORM/NestJS entities that add index signatures; enabling the new decorator proposal on code written for legacy decorators.

Related errors

• emitting decorator metadata while using new proposal
• swcify
• Babel doesn't support this right now.
• {decl:?}
• Decl in ExportDecl

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/38305c659aa554f8. Report an issue: GitHub.

Appendix: source

Thrown at crates/swc_ecma_transforms_proposal/src/decorators/mod.rs:557

                                                    span: DUMMY_SP,
                                                    arg: Some(value),
                                                })],
                                            }),
                                            ..Default::default()
                                        }
                                        .into(),
                                    }),
                                    _ => Prop::KeyValue(KeyValueProp {
                                        key: PropName::Ident(quote_ident!("value")),
                                        value: Expr::undefined(DUMMY_SP),
                                    }),
                                }))))
                                .collect(),
                            }
                            .as_arg(),
                        )
                    }
                    _ => unimplemented!("ClassMember::{:?}", member,),
                }
            })
            .map(Some)
            .collect();

        make_decorate_call(
            class.decorators,
            iter::once({
                // function(_initialize) {}
                Function {
                    span: DUMMY_SP,

                    params: iter::once(initialize.into())
                        .chain(super_class_ident.map(Pat::from))
                        .map(|pat| Param {
                            span: DUMMY_SP,
                            decorators: Vec::new(),
                            pat,

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