Reject an unparenthesized arrow function as the left operand of **

FIX: Reject uses of `**` with a non-parenthesized arrow function as left-hand side.

Author: spokodev

acorn/src/expression.js

@@ -269,7 +269,7 @@ pp.parseMaybeUnary = function(refDestructuringErrors, sawUnary, incDec, forInit)
     }
   }
 
-  if (!incDec && this.eat(tt.starstar)) {
+  if (!incDec && !(expr.type === "ArrowFunctionExpression" && expr.start === startPos) && this.eat(tt.starstar)) {
     if (sawUnary)
       this.unexpected(this.lastTokStart)
     else

test/tests-es7.js

@@ -396,3 +396,9 @@ testFail(
 testFail("'use strict'; function y(x = 1) { 'use strict' }",
          "Illegal 'use strict' directive in function with non-simple parameter list (1:14)",
          {ecmaVersion: 7})
+
+// An unparenthesized arrow function cannot be the left operand of `**`:
+// its left operand must be an UpdateExpression, and an arrow function is an
+// AssignmentExpression, so `() => {} ** x` is a SyntaxError (matches V8).
+testFail("() => {} ** 2", "Unexpected token (1:9)", {ecmaVersion: 7})
+testFail("a => {} ** b", "Unexpected token (1:8)", {ecmaVersion: 7})