swc-project/swc · error

ClassMember::{:?}

Error message

ClassMember::{:?}

What it means

The non-legacy decorator transform builds a descriptors array from every class member. It handles `Method`, `PrivateMethod` and `ClassProp`, skips `Empty`/`TsIndexSignature`, and marks `Constructor` unreachable; all remaining members — private properties (`#x = 1`), static blocks, TS parameter properties, auto-accessors — hit `_ => unimplemented!("ClassMember::...")`. Because it iterates every member, the mere presence of such a member in a decorated class panics even if that member has no decorator.

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,

View on GitHub (pinned to d7d7434666)

Solutions

1. Switch to `jsc.transform.legacyDecorator: true` for classes that mix decorators with `#` fields or static blocks
2. Replace `#x` private fields with plain (TS `private`) properties in decorated classes, and move static blocks out
3. Reduce to a minimal repro and report upstream — extending the match arm in decorators/mod.rs is a tractable contribution

Example fix

// before
 class C {
   #count = 0;
   @dec increment() { this.#count++; }
 }

// after
 class C {
   private count = 0; // or legacyDecorator: true
   @dec increment() { this.count++; }
 }

// Rust: scan classes for unsupported members before the non-legacy decorator pass
fn class_has_unsupported_members(c: &Class) -> bool {
    c.body.iter().any(|m| matches!(m,
        ClassMember::PrivateProp(..) | ClassMember::StaticBlock(..) | ClassMember::TsParamProp(..)))
}

fn decorator_safe_member(m: &ClassMember) -> bool {
    !matches!(m, ClassMember::PrivateProp(..) | ClassMember::StaticBlock(..) | ClassMember::TsParamProp(..))
}

When it happens

Trigger: Use modern (non-legacy) decorators on a class that also contains `#private` fields, `static { ... }` blocks, or TS constructor parameter properties.

Common situations: Decorator codebases that adopted `#` private state or static blocks before migrating off legacy decorators; Deno/TS-style classes mixing modern features; upgrading a legacy-decorator project that already used `#` fields.

Related errors

• emitting decorator metadata while using new proposal
• codegen of `export default from 'foo';`

AI-assisted analysis of swc-project/swc@d7d7434666 (2026-08-16). Data as JSON: /api/errors/38305c659aa554f8. Report an issue: GitHub.