Regression (6.2.0): styled.<tag>(fn) parameter (incl. theme) silently becomes `any` with @types/react ≤ 18.2.0

Summary

Since styled-components 6.2.0, the parameter of an interpolation/style function passed to a styled.<tag>(...) factory silently resolves to any when the project uses @types/react ≤ 18.2.0. This includes the theme property and any custom props declared via the generic. There is no compile error — even under strict — so typos and wrong types go completely undetected.

It worked correctly in 6.1.x and still works in 6.2.0+ if @types/react is bumped to ≥ 18.2.79, so the breakage is the interaction between styled-components' types (changed in 6.2.0) and older @types/react.

Minimal reproduction

package.json:

{
  "dependencies": {
    "react": "18.2.0",
    "react-dom": "18.2.0",
    "styled-components": "6.4.2"
  },
  "devDependencies": {
    "@types/react": "18.0.28",
    "@types/react-dom": "18.0.10",
    "typescript": "5.8.3"
  }
}

tsconfig.json: { "compilerOptions": { "strict": true, "jsx": "react-jsx", "moduleResolution": "node", "module": "commonjs", "target": "es2019", "lib": ["dom", "esnext"], "skipLibCheck": true, "noEmit": true, "esModuleInterop": true } }

import styled from 'styled-components';

// `$x` is declared as `number`. Assigning it to a `string` MUST error.
// If it does NOT error, the destructured parameter has silently become `any`.
export const Box = styled.div<{ $x: number }>(({ $x }) => {
  const leak: string = $x; // expected: TS2322. bug: no error -> `$x` is `any`
  return `content: "${leak}";`;
});

Run tsc --noEmit.

No DefaultTheme augmentation is required to reproduce — theme is affected identically (it resolves to any instead of DefaultTheme).

Expected

{ $x } is typed ($x: number, theme: DefaultTheme), so const leak: string = $x errors with TS2322: Type 'number' is not assignable to type 'string'.

Actual

With styled-components ≥ 6.2.0 and @types/react ≤ 18.2.0, the parameter is any, so the assignment compiles with no error. The failure is silent.

Bisect

TypeScript pinned to 5.8.3 throughout, repro file unchanged.

styled-components version (holding @types/[email protected]):

→ regression introduced in 6.2.0.

@types/react version (holding [email protected]):

→ fixed by @types/react ≥ 18.2.79.

Notes

• The standalone css helper (css`...${({ theme }) => ...}`) types the parameter correctly in all combinations; only the styled.<tag>(...) factory path regresses.
• The most concerning aspect is that it's silent — projects on older @types/react lose all prop/theme type-safety inside styled interpolations with no error or warning after upgrading past 6.1.x. If 6.2.0 began relying on a @types/react ≥ 18.2.x type feature, it would be worth either documenting a minimum @types/react or degrading more loudly than any.

Environment

• styled-components: 6.2.0 – 6.4.2 (regression starts at 6.2.0)
• @types/react: ≤ 18.2.0 (triggers); ≥ 18.2.79 (fixed)
• typescript: 5.8.3
• react: 18.2.0