better error for bind:value="{{foo}}" - fixes #437

src/parse/read/directives.js

@@ -77,6 +77,17 @@ export function readBindingDirective ( parser, start, name ) {
 
 		const a = parser.index;
 
+		if ( parser.eat( '{{' ) ) {
+			let message = 'bound values should not be wrapped';
+			const b = parser.template.indexOf( '}}', a );
+			if ( b !== -1 ) {
+				const value = parser.template.slice( parser.index, b );
+				message += ` — use '${value}', not '{{${value}}}'`;
+			}
+
+			parser.error( message, a );
+		}
+
 		// this is a bit of a hack so that we can give Acorn something parseable
 		let b;
 		if ( quoteMark ) {

test/parser/samples/error-binding-mustaches/error.json

@@ -0,0 +1,8 @@
+{
+	"message": "bound values should not be wrapped — use 'foo', not '{{foo}}'",
+	"loc": {
+		"line": 1,
+		"column": 19
+	},
+	"pos": 19
+}

test/parser/samples/error-binding-mustaches/input.html

@@ -0,0 +1 @@
+<input bind:value='{{foo}}'>