Here's a clearer case that Firefox gets wrong (or your grammar gets wrong,
depending on your point of view):
function f() {return "f"}
var x = 3;
let (a = 1) a ? f : x++();
The grammar states that the last statement must evaluate to "f". Firefox gives
a syntax error. This is incorrect because the let expression up to the ++ is a
PrimaryExpression so can be used as the left operand of a function call.
This is why ambiguous grammars are bad and unsuitable for our spec. In an
unambiguous grammar, if you find a series of production expansions that matches
a source program, then you know that those are the expansions that will be
taken. In an ambiguous grammar, you also need to prove the negative: no
*other* expansion can match that source program and shadow your expansions.
Proving the negative causes trouble because a language extension could turn the
mismatch into a match and because sometimes, as in the above case, you expected
some other part of the grammar to shadow your expansions but it didn't.
Waldemar
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