fix corner case in conditionals (#3244)

lib/compress.js

@@ -6048,14 +6048,15 @@ merge(Compressor.prototype, {
         //                   v
         // exp = foo ? something : something_else;
         var seq_tail = consequent.tail_node();
-        var alt_tail = alternative.tail_node();
+        if (seq_tail instanceof AST_Assign) {
-        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)
                 && alt_tail instanceof AST_Assign
                 && seq_tail.operator == alt_tail.operator
                 && seq_tail.left.equivalent_to(alt_tail.left)
                 && (!condition.has_side_effects(compressor)
-                || seq_tail.operator == "="
+                    || is_eq && !seq_tail.left.has_side_effects(compressor))) {
-                    && !seq_tail.left.has_side_effects(compressor))) {
                 return make_node(AST_Assign, self, {
                     operator: seq_tail.operator,
                     left: seq_tail.left,
@@ -6066,6 +6067,7 @@ merge(Compressor.prototype, {
                     })
                 });
             }
+        }
         // x ? y(a) : y(b) --> y(x ? a : b)
         var arg_index;
         if (consequent instanceof AST_Call

test/compress/conditionals.js

@@ -1364,3 +1364,23 @@ cond_seq_assign_2: {
         "42",
     ]
 }
+
+cond_seq_assign_3: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        var c = 0;
+        if (this)
+            c = 1 + c, c = c + 1;
+        else
+            c = 1 + c, c = c + 1;
+        console.log(c);
+    }
+    expect: {
+        var c = 0;
+        this, c = 1 + c, c += 1;
+        console.log(c);
+    }
+    expect_stdout: "2"
+}