sloppy-mode let more incompatible than spec lets on (due to automatic semi-colon insertion)

[saurik]

In ES3/5, the code x=true;eval("var let;let\nx=false");x evaluates to false, while in ES6 the same code evaluates to true, as in earlier versions of ECMAScript automatic-semicolon insertion intervenes and breaks what is now a LexicalDeclaration into two separate ExpressionStatements. This seems like something that should be documented in Annex E.

FWIW, I can see that 13.5 under Annex E tries to talk about the case of a StatementListItem which begins with "let [", but the incompatibility is actually more pervasive (though likely harmless). Also, I will point out that I can actually see someone formatting their (ECMAScript 6) code in an ambiguous way: putting variables on their own lines after the let, such as "let\n\tx=0,\n\ty=1".

Legacy ECMAScript:

V8 version 2.2.23
> x=true;eval("var let;let\nx=false");x
false

Rhino 1.7 release 3 2012 05 18
js> x=true;eval("var let;let\nx=false");x
false

JavaScriptCore (Mac OS X 10.9)
>>> x=true;eval("var let;let\nx=false");x
false

ECMAScript 2015:

SpiderMonkey 5bcd1d1b 2015/12/05
js> x=true;eval("var let;let\nx=false");x
true

[wycats]

@saurik what do you think is the solution?

[bterlson]

This is a great catch. Right thing to do is document in Annex E:

13.2: In ECMAScript 2015, a |StatementList| beginning with the token let followed by the input elements |LineTerminator| |Identifier| is the start of a |LexicalDeclaration|. In previous editions, automatic semicolon insertion would always insert a semicolon before the |Identifier| input element.