fix: only throw bind error when not passing a value (#10090)

Relax the runtime error to only throw when you're passing a binding with an undefined value. This makes it possible to provide components in a way that can be used more flexibly while keeping the error to guard against the case that we want to avoid: a default value propagation up.

Author: dummdidumm

.changeset/big-geese-act.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: only throw bind error when not passing a value

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

@@ -1567,16 +1567,20 @@ export function is_store(val) {
 export function prop(props, key, flags, initial) {
 	var immutable = (flags & PROPS_IS_IMMUTABLE) !== 0;
 	var runes = (flags & PROPS_IS_RUNES) !== 0;
-
-	var setter = get_descriptor(props, key)?.set;
-	if (DEV && setter && runes && initial !== undefined) {
-		// TODO consolidate all these random runtime errors
-		throw new Error('Cannot use fallback values with bind:');
-	}
-
 	var prop_value = /** @type {V} */ (props[key]);
+	var setter = get_descriptor(props, key)?.set;
 
 	if (prop_value === undefined && initial !== undefined) {
+		if (setter && runes) {
+			// TODO consolidate all these random runtime errors
+			throw new Error(
+				'ERR_SVELTE_BINDING_FALLBACK' +
+					(DEV
+						? `: Cannot pass undefined to bind:${key} because the property contains a fallback value. Pass a different value than undefined to ${key}.`
+						: '')
+			);
+		}
+
 		// @ts-expect-error would need a cumbersome method overload to type this
 		if ((flags & PROPS_IS_LAZY_INITIAL) !== 0) initial = initial();
 

packages/svelte/tests/runtime-runes/samples/props-bound-fallback/_config.js

@@ -14,5 +14,5 @@ export default test({
 		assert.htmlEqual(target.innerHTML, `<button>1</button><span>1</span>`);
 	},
 
-	error: `Cannot use fallback values with bind:`
+	error: `ERR_SVELTE_BINDING_FALLBACK: Cannot pass undefined to bind:count because the property contains a fallback value. Pass a different value than undefined to count.`
 });