override val isData: Boolean
        get() = withValidityAssertion { descriptor.isData }

    override val isInline: Boolean
        get() = withValidityAssertion { descriptor.isInline || descriptor.isValue }

    override val isFun: Boolean
        get() = withValidityAssertion { descriptor.isFun }

    override val isExternal: Boolean
        get() = withValidityAssertion { descriptor.isExternal }

            // when is an `else`.
            is KtWhenExpression ->
                parent.subjectExpression == child && parent.entries.firstOrNull()?.isElse == false

            // Throw expressions use the expression thrown.
            is KtThrowExpression ->
                parent.thrownExpression == child