nullish coalescing operator missing `var` declaration when using spreads

[freben]

Describe the bug

It seems that since version 1.15.4, under just some circumstances, the temporary variable used in ?? nullish coalescing doesn't get declared, leading to errors in the resulting code.

Input code

export default function DropdownNavbarItemDesktop({
  items,
  ...props
}: Props): ReactNode {
  return (
    <div>
      <NavbarNavLink
        onClick={props.to ? undefined : (e) => e.preventDefault()}
      >
        {props.children ?? props.label}
      </NavbarNavLink>
    </div>
  );
}

Config

N/A

Link to the code that reproduces this issue

https://play.swc.rs/?version=1.15.5&code=H4sIAAAAAAAAA1WOwQqCQBCG7z7Ff1QIu5vpIS9BSPQGmzvR4jYr62hB%2BO5taUGX5dvhn29%2BenTOCzRd1GAFl4EbMY5Reddpd%2BdajWfl90K3ivpWXBc%2FI8CEf78KkKZpF5J9NGU4viHJcCLVSO004R31JINnxAGBXJux%2BFDgWR3eg%2BF2GQKOd9Y07fb58abiUGLg0M8waWSIKcG2AIW7NBJLNTePk2lRFD%2FVomiuxmpPjLLEPLHqTPabz9d%2FReb1fL00TTbRFL0AHLPgWiUBAAA%3D&config=H4sIAAAAAAAAA1WPSw7CMAxE9z1F5DVbWHAHDhEFtwrKT7ErEVW9O%2FkDu%2FjNTMY%2BFiHgRQru4sjPPAQZCeOcM6HkWL4zAU4BSUUdGC5DZSoSxx0rOZsALOOGXEJI1%2B4G4z1hZqs0hJ1Z7fSafvuUtyEi0b%2BxWKXbzMy3uqVXgvXPvYr9jrJrq7%2FB1zTK5seg6TGS9YjzA2yuiPwTAQAA

SWC Info output

    Operating System:
        Platform: darwin
        Arch: arm64
        Machine Type: arm64
        Version: Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:40 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T6000
        CPU: (10 cores)
            Models: Apple M1 Max

    Binaries:
        Node: 22.18.0
        npm: 10.9.3
        Yarn: 4.8.1
        pnpm: N/A

    Relevant Packages:
        @swc/core: 1.15.5
        @swc/helpers: 0.5.17
        @swc/types: 0.1.25
        typescript: 5.7.3

    SWC Config:
        output: N/A
        .swcrc path: N/A

    Next.js info:
        output: N/A

Expected behavior

// props.children ?? props.label
var _props_children;
(_props_children = props.children) !== null && _props_children !== void 0 ? _props_children : props.label

This was working in 1.15.3.

Actual behavior

// props.children ?? props.label
(_props_children = props.children) !== null && _props_children !== void 0 ? _props_children : props.label

The temporary variable that created for the initial LHS assignment for the ?? operator never gets declared.

This worked correctly in 1.15.3, but is broken in 1.15.4 and newer.

Version

1.15.4

Additional context

No response

[freben]

Possibly related error reported by another user:

 FAIL   @drodil/backstage-plugin-qeta-react  plugins/qeta-react/src/hooks/useVoting.test.tsx
  ● Test suite failed to run

    ReferenceError: _n_source_codePointAt is not defined

      10 | import { qetaApiRef } from '../api';
      11 | import { errorApiRef } from '@backstage/core-plugin-api';
    > 12 | import { translationApiRef } from '@backstage/core-plugin-api/alpha';
         |                    ^
      13 |
      14 | jest.mock('@backstage/plugin-signals-react', () => ({
      15 |   useSignal: jest.fn(),

I thought they could be related since it's also seemingly a failure to declare a temp var. This happened when bumping to react 30.

[kdy1]

I'll publish a new version of @swc/core once #11377 is merged

[kdy1]

v1.15.6: https://github.com/swc-project/swc/actions/runs/20323853689

[freben]

Thank you!