Using styles from a class that has been defined using an ampersand

Question

Sorry about the title, I'm ignorant of the precise terms to use.

This is the problem:

.flex{
    display: flex;

    &--grow{
        flex-grow: grow;
    }
}

I want to use the .flex--grow class in another class (extend the styles? not sure about the correct wording here).

.nice{
    .flex--grow;
}

This would normally work, but because the .flex--grow class is defined in a nested form, it doesn't. Is there a way around this, retaining the nested nature of it?