How do I use /deep/ or >>> or ::v-deep in Vue.js?

So, I've read here that in Vue.js, you can use /deep/ or >>> in a selector in order to create style rules that apply to elements inside of child components. However, attempting to use this in my styles, whether in SCSS or in plain old CSS, doesn't work. Instead, they are sent to the browser verbatim, and therefore have no effect. For example:

home.vue:

<style lang="css" scoped>
.autocomplete >>> .autocomplete-input 
{
// ...
}
</style>

generated css:

<style type="text/css">
.autocomplete >>> .autocomplete-input[data-v-2bda0c9a]
{
//...
}
</style>

what I want:

<style type="text/css">
.autocomplete[data-v-2bda0c9a] .autocomplete-input
{
//...
}
</style>

My webpack configuration pertaining to vue-loader looks like this:

// ...
{
    test: /\.vue$/,
    loader: "vue-loader",
    options: {
        loaders: {
            scss: "vue-style-loader!css-loader!sass-loader"
        }
    }
}
// ...

So my question is, how do I get this >>> operator to work?

I've already found this answer, but I'm doing exactly that and it doesn't work...

Vue 2

^{The following also works in Vue 3 but is deprecated.}

Sass:   Use ::v-deep

::v-deep .child-class {
    background-color: #000;
}

Not using Sass:   Use >>>

>>> .child-class {
    background-color: #000;
}

With either syntax, the <style> tag for this component must be scoped:

<style scoped>

Vue 3 (Updated 1/19/2022)

In Vue 3, the ::v- prefix is now deprecated and we no longer need >>>. We can use the unified :deep selector whether using Sass or not, but now it's recommended to use parentheses with the selector.

Use :deep(.child-class)

:deep(.child-class) {
    background-color: #000;
}

^{You can also still use any of the deprecated syntaxes if you prefer.}

Vue 3 new selectors

Additionally, in Vue 3, there is a new feature for components with a <slot> that enables styling passed slot content.

Slot content   Use :slotted(.child-class)

:slotted(.slot-class) {
    background-color: #000;
}

And lastly, in Vue 3, there is a new feature to register global styles even from a scoped component:

Global styles   Use :global(.my-class)

:global(.my-class) {
    background-color: #000;
}

With any syntax, the <style> tag for this component must be scoped:

<style scoped>

Summary

In Vue 2:

• The /deep/ syntax is deprecated
• Use ::v-deep with Sass, use >>> without Sass

In Vue 3:

• ::v-deep .child-class is deprecated in favor of :deep(.child-class)
• The ::v- prefix is deprecated in favor of :
• The >>> syntax is deprecated
• The /deep/ syntax is deprecated
• There are new :slotted and :global selectors

With every version/syntax, the <style> tag for this component must be scoped:

<style scoped>

Avoid using /deep/ and instead use ::v-deep

Any scoped component's css can be changed by using deep selector but sooner /deep/ will get deprecated

Vue Github reference - https://github.com/vuejs/vue-loader/issues/913

Use ::v-deep in this case,and avoid deprecated /deep/

Reference - Deep Selector

Just inspect class of the rendered element which you want to modify using devtools in chrome or any browser console.

Then, In you consuming component, modify it

<style scoped>
::v-deep .custom-component-class {
   background: red; //
}
</style>

I've successfully used /deep/ in Vue's scoped SCSS stylesheets with this structure:

.parent-class {
  & /deep/ .child-class {
    background-color: #000;
  }
}

Edit: /deep/ is being deprecated, if it no longer works for you please refer to the other answer that explains ::v-deep

Though it is not found in the Documentation, the answer is that the component you are attempting to reach cannot be the root component. Wrap your single component in a <div> and it should work using ::v-deep on scoped scss as others have explained.