1 files changed, 12 insertions, 14 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 9201d890..f4aa62d4 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -610,6 +610,11 @@ merge(Compressor.prototype, {
 
         function is_immutable(value) {
             if (!value) return false;
+            if (value instanceof AST_Assign) {
+                var op = value.operator;
+                return op == "=" ? is_immutable(value.right) : !lazy_op[op.slice(0, -1)];
+            }
+            if (value instanceof AST_Sequence) return is_immutable(value.tail_node());
             return value.is_constant() || is_lambda(value) || value instanceof AST_ObjectIdentity;
         }
 
@@ -901,6 +906,7 @@ merge(Compressor.prototype, {
 
             function walk_assign() {
                 right.walk(tw);
+                var modified = is_modified(compressor, tw, node, right, 0, is_immutable(right), recursive_ref(tw, d));
                 scan_declaration(tw, compressor, left, function() {
                     return node.right;
                 }, function(sym, fixed, walk) {
@@ -911,10 +917,7 @@ merge(Compressor.prototype, {
                     }
                     var d = sym.definition();
                     d.assignments++;
-                    if (fixed
-                        && !is_modified(compressor, tw, node, right, 0, is_immutable(right), recursive_ref(tw, d))
-                        && !sym.in_arg
-                        && safe_to_assign(tw, d)) {
+                    if (fixed && !modified && !sym.in_arg && safe_to_assign(tw, d)) {
                         push_ref(d, sym);
                         mark(tw, d);
                         if (d.single_use && left instanceof AST_Destructured) d.single_use = false;
@@ -6299,18 +6302,13 @@ merge(Compressor.prototype, {
                     var drop_sym = is_var ? can_drop_symbol(def.name) : is_safe_lexical(sym);
                     if (!drop_sym || !drop_vars || sym.id in in_use_ids) {
                         if (value && indexOf_assign(sym, def) < 0) {
-                            var write_only = value.write_only;
                             value = value.drop_side_effect_free(compressor);
-                            if (def.value !== value) {
-                                if (value) {
-                                    AST_Node.warn("Side effects in last use of variable {name} [{file}:{line},{col}]", template(def.name));
-                                    side_effects.push(value);
-                                }
-                                value = null;
-                                trim_defns.push(def);
-                            } else if (value.write_only !== write_only) {
-                                value.write_only = write_only;
+                            if (value) {
+                                AST_Node.warn("Side effects in last use of variable {name} [{file}:{line},{col}]", template(def.name));
+                                side_effects.push(value);
                             }
+                            value = null;
+                            trim_defns.push(def);
                         }
                         var old_def;
                         if (!value && !(node instanceof AST_Let)) {