All the examples of using defineModel are with <script setup>. What if I want to use defineModel inside defineComponent?

It seems to be possible, but I’m struggling with setting the default model value. When I try this,
in parent component:

<Child v-model="foo" /> (foo is "1")

in child component:

<script>
import { defineComponent, defineModel } from 'vue'
export default defineComponent({
 setup(props, ctx) {
 const model1 = defineModel()
 console.log(model1) // returns "undefined" here and when used in template

 console.log(ctx.attrs.modelValue) // returns "1"
 const model2 = defineModel({ type: String, default: ctx.attrs.modelValue })
 console.log(model2) // returns "undefined" here and when used in template

 return { model1, model2 }
 }
})
</script>

I tried two approaches (model1 and model2) but both don’t work.

Why is model1 and model2 not picking up the default value from the modelValue fallthrough attribute (passed from parent to child with v-model="foo")? I’m on Vue v.3.4.23.