E0285: generic arrow function needs ',' here in TSX

In TypeScript, generic arrow functions have syntax like <P1, P2>(params) => body. In JSX, elements have syntax like <Element>{children}</Element>. Because of a possible ambiguity, generic arrow functions in TypeScript+JSX must have at least one comma inside the parameter list:

const reversed = <T>(array: T[]) =>
  [...array].reverse();

To fix this error, add a comma after the generic parameter:

const reversed = <T,>(array: T[]) =>
  [...array].reverse();

Alternatively, write a normal function instead of an arrow function:

function reversed<T>(array: T[]) {
  return [...array].reverse();
}

Introduced in quick-lint-js version 2.9.0.

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