Disallow arrow functions to be the conditional in a ternary expr

FIX: Don't allow the conditional in a ternary expression to be a (naked) arrow function.

Closes #1428

Author: marijnh

acorn/src/expression.js

@@ -167,7 +167,7 @@ pp.parseMaybeConditional = function(forInit, refDestructuringErrors) {
   let startPos = this.start, startLoc = this.startLoc
   let expr = this.parseExprOps(forInit, refDestructuringErrors)
   if (this.checkExpressionErrors(refDestructuringErrors)) return expr
-  if (this.eat(tt.question)) {
+  if (!(expr.type == "ArrowFunctionExpression" && expr.start == startPos) && this.eat(tt.question)) {
     let node = this.startNodeAt(startPos, startLoc)
     node.test = expr
     node.consequent = this.parseMaybeAssign()

test/tests-harmony.js

@@ -2637,6 +2637,8 @@ test("foo((x, y) => {})", {
   locations: true
 });
 
+testFail("() => {} ? 1 : 0", "Unexpected token (1:9)", {ecmaVersion: 6})
+
 // ES6: Method Definition
 
 test("x = { method() { } }", {