throw compiler error when binding directly to const variables (#4506)

src/compiler/compile/nodes/Binding.ts

@@ -72,6 +72,11 @@ export default class Binding extends Node {
 			});
 
 			variable[this.expression.node.type === 'MemberExpression' ? 'mutated' : 'reassigned'] = true;
+
+			if (info.expression.type === 'Identifier' && !variable.writable) component.error(this.expression.node, {
+				code: 'invalid-binding',
+				message: 'Cannot bind to a variable which is not writable',
+			});
 		}
 
 		const type = parent.get_static_attribute_value('type');

test/validator/samples/binding-const-field/errors.json

@@ -0,0 +1 @@
+[]

test/validator/samples/binding-const-field/input.svelte

@@ -0,0 +1,7 @@
+<script>
+	const dummy = {
+		foo: 'bar'
+	};
+</script>
+
+<input bind:value={dummy.foo}>

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

@@ -0,0 +1,15 @@
+[{
+	"code": "invalid-binding",
+	"message": "Cannot bind to a variable which is not writable",
+	"pos": 61,
+	"start": {
+		"line": 5,
+		"column": 19,
+		"character": 61
+	},
+	"end": {
+		"line": 5,
+		"column": 24,
+		"character": 66
+	}
+}]

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

@@ -0,0 +1,5 @@
+<script>
+	const dummy = 'foo';
+</script>
+
+<input bind:value={dummy}>