add async_derived_orphan error

Author: Rich-Harris

documentation/docs/98-reference/.generated/client-errors.md

@@ -1,5 +1,17 @@
 <!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
 
+### async_derived_orphan
+
+```
+Cannot create a `$derived(...)` with an `await` expression outside of an effect tree
+```
+
+In Svelte there are two types of reaction — [`$derived`](https://svelte.dev/docs/svelte/$derived) and [`$effect`](https://svelte.dev/docs/svelte/$effect). Deriveds can be created anywhere, because they run _lazily_ and can be [garbage collected](https://developer.mozilla.org/en-US/docs/Glossary/Garbage_collection) if nothing references them. Effects, by contrast, keep running eagerly whenever their dependencies change, until they are destroyed.
+
+Because of this, effects can only be created inside other effects (or [effect roots](https://svelte.dev/docs/svelte/$effect#$effect.root), such as the one that is created when you first mount a component) so that Svelte knows when to destroy them.
+
+Some sleight of hand occurs when a derived contains an `await` expression: Since waiting until we read `{await getPromise()}` to call `getPromise` would be too late, we use an effect to instead call it proactively, notifying Svelte when the value is available. But since we're using an effect, we can only create asynchronous deriveds inside another effect.
+
 ### bind_invalid_checkbox_value
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -1,3 +1,13 @@
+## async_derived_orphan
+
+> Cannot create a `$derived(...)` with an `await` expression outside of an effect tree
+
+In Svelte there are two types of reaction — [`$derived`](https://svelte.dev/docs/svelte/$derived) and [`$effect`](https://svelte.dev/docs/svelte/$effect). Deriveds can be created anywhere, because they run _lazily_ and can be [garbage collected](https://developer.mozilla.org/en-US/docs/Glossary/Garbage_collection) if nothing references them. Effects, by contrast, keep running eagerly whenever their dependencies change, until they are destroyed.
+
+Because of this, effects can only be created inside other effects (or [effect roots](https://svelte.dev/docs/svelte/$effect#$effect.root), such as the one that is created when you first mount a component) so that Svelte knows when to destroy them.
+
+Some sleight of hand occurs when a derived contains an `await` expression: Since waiting until we read `{await getPromise()}` to call `getPromise` would be too late, we use an effect to instead call it proactively, notifying Svelte when the value is available. But since we're using an effect, we can only create asynchronous deriveds inside another effect.
+
 ## bind_invalid_checkbox_value
 
 > Using `bind:value` together with a checkbox input is not allowed. Use `bind:checked` instead

packages/svelte/src/internal/client/errors.js

@@ -389,4 +389,20 @@ export function state_unsafe_mutation() {
 	} else {
 		throw new Error(`https://svelte.dev/e/state_unsafe_mutation`);
 	}
+}
+
+/**
+ * Cannot create a `$derived(...)` with an `await` expression outside of an effect tree
+ * @returns {never}
+ */
+export function async_derived_orphan() {
+	if (DEV) {
+		const error = new Error(`async_derived_orphan\nCannot create a \`$derived(...)\` with an \`await\` expression outside of an effect tree\nhttps://svelte.dev/e/async_derived_orphan`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/async_derived_orphan`);
+	}
 }

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -100,7 +100,7 @@ export function async_derived(fn, location) {
 	let parent = /** @type {Effect | null} */ (active_effect);
 
 	if (parent === null) {
-		throw new Error('TODO cannot create unowned async derived');
+		e.async_derived_orphan();
 	}
 
 	var boundary = /** @type {Boundary} */ (parent.b);