better error for bindings to let: values - fixes #2301

src/compile/nodes/Binding.ts

@@ -31,7 +31,12 @@ export default class Binding extends Node {
 		this.is_contextual = scope.names.has(name);
 
 		// make sure we track this as a mutable ref
-		if (this.is_contextual) {
+		if (scope.is_let(name)) {
+			component.error(this, {
+				code: 'invalid-binding',
+				message: 'Cannot bind to a variable declared with the let: directive'
+			});
+		} else if (this.is_contextual) {
 			scope.dependencies_for_name.get(name).forEach(name => {
 				const variable = component.var_lookup.get(name);
 				variable[this.expression.node.type === 'MemberExpression' ? 'mutated' : 'reassigned'] = true;

test/validator/samples/binding-let/errors.json

@@ -0,0 +1,15 @@
+[{
+	"code": "invalid-binding",
+	"message": "Cannot bind to a variable declared with the let: directive",
+	"pos": 52,
+	"start": {
+		"line": 6,
+		"column": 8,
+		"character": 52
+	},
+	"end": {
+		"line": 6,
+		"column": 24,
+		"character": 68
+	}
+}]

test/validator/samples/binding-let/input.svelte

@@ -0,0 +1,7 @@
+<script>
+	let Foo;
+</script>
+
+<Foo let:bar>
+	<input bind:value={bar}>
+</Foo>