When exactly is the operator keyword required in Kotlin?

Why does this code compile?

This compiles because the overridden interface method, Comparable<T>.compareTo, is itself an operator fun.

/**
 * Compares this object with the specified object for order. Returns zero if this object is equal
 * to the specified [other] object, a negative number if it's less than [other], or a positive number
 * if it's greater than [other].
 */
public operator fun compareTo(other: T): Int

As the function overrides this, it is also an operator function.

When exactly is operator required?

operator in general is required whenever you wish to be able to use a function as if it were an operator, since operator usages are simply compiled to function calls (except on primitive types, etc.)

That is, foo += bar, for example, is equivalent to foo.plusAssign(bar), foo[bar] = baz is equivalent to foo.set(bar, baz), etc.

Personally I prefer specifying operator wherever possible even if it is not required, for readability reasons.

If MyDate were not a Comparable, and you omitted the operator modifier, comparing two dates via <, >, <=, or >= would not compile.

I couldn’t find anything in the specification on this, though. However in a polymorphic sense it makes sense – why should you be able to write a < b where the type of a and b are Comparables, but not when they are a MyDate? Since you wouldn’t be able to remove the “operator-ness” of this function, it makes sense that operator should be inheritable from the superclass method.