<aura:component implements="force:appHostable">

    <aura:attribute name="mystr" type="String" required="false" default="hello world" access="private" description="" />

    <aura:attribute name="mycmp" type="Aura.Component[]" access="public" description="test">
        <span>{! v.mystr }</span>
    </aura:attribute>

</aura:component>

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Error during init [Assertion Failed!: Unable to get value for key 'v.mystr'. No value provider was found for 'v'. : false]