dev mode errors for non-stores

src/compile/render-dom/index.ts

@@ -287,6 +287,7 @@ export default function dom(
 	const reactive_store_subscriptions = reactive_stores.length > 0 && reactive_stores
 		.map(name => deindent`
 			let ${name};
+			${component.options.dev && `@validate_store(${name.slice(1)}, '${name.slice(1)}');`}
 			$$self.$$.on_destroy.push(${name.slice(1)}.subscribe($$value => { ${name} = $$value; $$make_dirty('${name}'); }));
 		`)
 		.join('\n\n');

src/compile/render-ssr/index.ts

@@ -37,7 +37,13 @@ export default function ssr(
 	}
 
 	const reactive_stores = Array.from(component.template_references).filter(n => n[0] === '$');
-	const reactive_store_values = reactive_stores.map(name => `const ${name} = @get_store_value(${name.slice(1)});`)
+	const reactive_store_values = reactive_stores.map(name => {
+		const assignment = `const ${name} = @get_store_value(${name.slice(1)});`;
+
+		return component.options.dev
+			? `@validate_store(${name.slice(1)}, '${name.slice(1)}'); ${assignment}`
+			: assignment;
+	});
 
 	// TODO only do this for props with a default value
 	const parent_bindings = component.javascript

src/internal/utils.js

@@ -55,4 +55,10 @@ export function safe_not_equal(a, b) {
 
 export function not_equal(a, b) {
 	return a != a ? b == b : a !== b;
+}
+
+export function validate_store(store, name) {
+	if (!store || typeof store.subscribe !== 'function') {
+		throw new Error(`'${name}' is not a store with a 'subscribe' method`);
+	}
 }

test/runtime/samples/store-dev-mode-error/_config.js

@@ -0,0 +1,11 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+
+	props: {
+		count: 0
+	},
+
+	error: `'count' is not a store with a 'subscribe' method`
+};

test/runtime/samples/store-dev-mode-error/main.html

@@ -0,0 +1,5 @@
+<script>
+	export let count;
+</script>
+
+<button on:click="{() => count.update(n => n + 1)}">count {$count}</button>