E0238: assignment-asserted fields are not supported in interfaces

In TypeScript, ! after a field name indicates a definite assignment assertion. These assertions only make sense for classes, not for interfaces. It is a syntax error to write a definite assignment assertion on an interface field:

interface Point2D {
  x!: number;
  y!: number;
}

To fix this error, remove the !:

interface Point2D {
  x: number;
  y: number;
}

Introduced in quick-lint-js version 2.6.0.

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