swc-project/swc · error

flow type cast

Error message

flow type cast

What it means

The estree TypeCastExpression is Flow's `(value as T)`-style cast (Babel Flow output). SWC's AST has no Flow cast expression (only TsAsExpr for TS), so Swcify for TypeCastExpression is declared with Output = Never and immediately panics with `unimplemented!("flow type cast")`.

Solutions

1. Run Flow type stripping (@babel/plugin-transform-flow-strip-types) on the AST/source before swcify
2. Parse the code as TypeScript when the casts are actually TS `as` casts
3. Drop or replace TypeCastExpression nodes with their inner expression in a pre-conversion pass

Example fix

// before
const n = (x: number); // Flow cast -> TypeCastExpression -> panic

// after
const n = x; // after flow-strip-types, or parse as TS: x as number

// Node: strip Flow types before swcify
const stripped = require('@babel/core').transformSync(code, {
  plugins: ['@babel/plugin-transform-flow-strip-types'],
  code: true,
}).code;

function isFlowTypeCast(node) {
  return node.type === 'TypeCastExpression';
}

When it happens

Trigger: Running swcify on a Babel AST produced by a Flow-mode parse that contains Flow type cast expressions, e.g. `(x: MyType)` or `(x >>> null)`-style Flow annotations.

Common situations: Converting Flow codebases (legacy React Native, old Facebook packages) through Babel into SWC-based tooling without first stripping Flow types.

Related errors

• flow
• flow types
• Babel doesn't support this right now.
• failed to swcify arguments
• flow type

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/58e8fdce493d6534. Report an issue: GitHub.

Appendix: source

Thrown at crates/swc_estree_compat/src/swcify/expr.rs:842

        OptChainExpr {
            span: ctx.span(&self.base),
            optional: self.optional,
            base: Box::new(OptChainBase::Call(OptCall {
                span: ctx.span(&self.base),
                callee: self.callee.swcify(ctx),
                args: self.arguments.swcify(ctx).into_iter().flatten().collect(),
                type_args: self.type_parameters.swcify(ctx).map(From::from),
                ..Default::default()
            })),
        }
    }
}

impl Swcify for TypeCastExpression {
    type Output = Never;

    fn swcify(self, _: &Context) -> Self::Output {
        unimplemented!("flow type cast")
    }
}

impl Swcify for swc_estree_ast::JSXElement {
    type Output = swc_ecma_ast::JSXElement;

    fn swcify(self, ctx: &Context) -> Self::Output {
        swc_ecma_ast::JSXElement {
            span: ctx.span(&self.base),
            opening: self.opening_element.swcify(ctx),
            children: self.children.swcify(ctx),
            closing: self.closing_element.swcify(ctx),
        }
    }
}

impl Swcify for swc_estree_ast::JSXOpeningElement {
    type Output = swc_ecma_ast::JSXOpeningElement;

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