fix chained evaluation

[alexlamsl]

reduce_vars enables substitution of variables but did not clone the value's AST_Node.

This confuses collapse_vars and result in invalid AST and subsequent crash.

fixes #1609

[kzc]

This looks odd. collapse_vars would not make this substitution of f(b) -> f(a) because function f is undeclared and would be a side effect. There might be still another clone issue in reduce_vars in var b of value a.

v2.7.5 by comparison produces:

$ uglifyjs chained_evaluation_2.js -c collapse_vars,reduce_vars -b
WARN: Replacing constant a [chained_evaluation_2.js:4,24]
!function() {
    !function() {
        var c, b = "long piece of string";
        c = f(b), c.bar = b;
    }();
}();

[kzc]

@alexlamsl Did reduce_vars eliminate var b altogether and replace all its references with a? If so, then it makes sense.

[kzc]

Indeed - with this PR or 2.8.12:

$ bin/uglifyjs chained_evaluation_2.js -c collapse_vars=false,reduce_vars=true -b
WARN: Dropping unused variable b [chained_evaluation_2.js:4,20]
!function() {
    var a = "long piece of string";
    !function() {
        var c;
        c = f(a), c.bar = a;
    }();
}();

[kzc]

It's fine.

[alexlamsl]

Yes, you need both reduce_vars and collapse_vars to trigger this issue.

It's reduce_vars fault, because it didn't clone the AST_Node when performing the substitution, then collapse_vars operated on the resulting invalid AST and gave us AST_Binary.right === null.

[kzc]

Travis CI is erring out on installing node 4:

https://travis-ci.org/mishoo/UglifyJS2/jobs/211675141

[alexlamsl]

Travis CI is erring out on installing node 4:

See #1604 (comment)

(Sorry I was away playing badminton.)