fix corner case in conditionals

lib/compress.js

@@ -6051,12 +6051,14 @@ merge(Compressor.prototype, {
         if (seq_tail instanceof AST_Assign) {
             var is_eq = seq_tail.operator == "=";
             var alt_tail = is_eq ? alternative.tail_node() : alternative;
-            if ((is_eq || consequent instanceof AST_Assign)
+            if ((is_eq || consequent === seq_tail)
                 && alt_tail instanceof AST_Assign
                 && seq_tail.operator == alt_tail.operator
                 && seq_tail.left.equivalent_to(alt_tail.left)
-                && (!condition.has_side_effects(compressor)
-                    || is_eq && !seq_tail.left.has_side_effects(compressor))) {
+                && (is_eq && !seq_tail.left.has_side_effects(compressor)
+                    || !condition.has_side_effects(compressor)
+                        && can_shift_lhs_of_tail(consequent)
+                        && can_shift_lhs_of_tail(alternative))) {
                 return make_node(AST_Assign, self, {
                     operator: seq_tail.operator,
                     left: seq_tail.left,
@@ -6221,6 +6223,19 @@ merge(Compressor.prototype, {
             }
         }
 
+        function can_shift_lhs_of_tail(node) {
+            if (node === node.tail_node()) return true;
+            var exprs = node.expressions;
+            for (var i = exprs.length - 1; --i >= 0;) {
+                var expr = exprs[i];
+                if (!(expr instanceof AST_Assign) && expr.has_side_effects(compressor)
+                    || expr.operator != "="
+                    || expr.left.has_side_effects(compressor)
+                    || expr.right.has_side_effects(compressor)) return false;
+            }
+            return true;
+        }
+
         function pop_lhs(node) {
             if (!(node instanceof AST_Sequence)) return node.right;
             var exprs = node.expressions.slice();

test/compress/conditionals.js

@@ -1384,3 +1384,34 @@ cond_seq_assign_3: {
     }
     expect_stdout: "2"
 }
+
+issue_3271: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        function f(a) {
+            var i = 0, b = [];
+            if (a) {
+                b[i++] = 4,
+                b[i++] = 1;
+            } else {
+                b[i++] = 3,
+                b[i++] = 2,
+                b[i++] = 1;
+            }
+            return b;
+        }
+        console.log(f(0).pop(), f(1).pop());
+    }
+    expect: {
+        function f(a) {
+            var i = 0, b = [];
+            a ? b[i++] = 4 : (b[i++] = 3, b[i++] = 2),
+            b[i++] = 1;
+            return b;
+        }
+        console.log(f(0).pop(), f(1).pop());
+    }
+    expect_stdout: "1 1"
+}