Better fix for #343

We can in fact lift sequences, but only if the operation is assignment and
the left-hand side has no side effects nor property access -- that should
guarantee that whatever we place before it cannot affect the sense of the
assignment.

Dropped contrived test case (too hard to support it now), added a more
meaningful one.

1 files changed, 18 insertions, 0 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 59caa15c..f44277cd 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1865,6 +1865,14 @@ merge(Compressor.prototype, {
         return self.evaluate(compressor)[0];
     });
 
+    function has_side_effects_or_prop_access(node, compressor) {
+        var save_pure_getters = compressor.option("pure_getters");
+        compressor.options.pure_getters = false;
+        var ret = node.has_side_effects(compressor);
+        compressor.options.pure_getters = save_pure_getters;
+        return ret;
+    }
+
     AST_Binary.DEFMETHOD("lift_sequences", function(compressor){
         if (compressor.option("sequences")) {
             if (this.left instanceof AST_Seq) {
@@ -1875,6 +1883,16 @@ merge(Compressor.prototype, {
                 seq = AST_Seq.from_array(x).transform(compressor);
                 return seq;
             }
+            if (this.right instanceof AST_Seq
+                && this instanceof AST_Assign
+                && !has_side_effects_or_prop_access(this.left, compressor)) {
+                var seq = this.right;
+                var x = seq.to_array();
+                this.right = x.pop();
+                x.push(this);
+                seq = AST_Seq.from_array(x).transform(compressor);
+                return seq;
+            }
         }
         return this;
     });