[haskou]

What

FileSize.add and FileSize.subtract were typed as NumberValueObject (inherited), although at runtime they return a FileSize. They are now declared as add(other: FileSize): FileSize and subtract(other: FileSize): FileSize.

Notes

• Adding or subtracting two FileSize is a closed operation, so the return type is exact. NumberValueObject is unchanged: its arithmetic can fail on subclass invariants, so this would be a wrong promise there.
• Implemented with clone(), so subclasses of FileSize keep their own type at runtime.
• Breaking at type level: size.add(100) with a plain number no longer compiles. Pass new FileSize(100).
• Docs for FileSize updated with the new signatures and the negative-result error.

Verification

yarn test, yarn build pass. Coverage stays at 100%. The arithmetic test calls FileSize.to() on the results, so it only compiles if both return FileSize.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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