fix corner case in unused (#5080)

fixes #5079

lib/compress.js

@@ -6366,7 +6366,10 @@ merge(Compressor.prototype, {
                     if (value) {
                         if (props.assign) {
                             var assign = props.assign.drop_side_effect_free(compressor);
-                            if (assign) props.unshift(assign);
+                            if (assign) {
+                                assign.write_only = true;
+                                props.unshift(assign);
+                            }
                         }
                         if (!(parent instanceof AST_Sequence)
                             || parent.tail_node() === node
@@ -6942,7 +6945,6 @@ merge(Compressor.prototype, {
                 if (assign) {
                     assign.write_only = true;
                     assign.walk(tw);
-                    assign.write_only = "p";
                 }
                 props.forEach(function(prop) {
                     prop.walk(tw);

test/compress/drop-unused.js

@@ -3469,3 +3469,26 @@ issue_4912_3: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5079: {
+    options = {
+        collapse_vars: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        do {
+            (a = 123456).p = a;
+            a.q = null;
+        } while (console.log("PASS"));
+    }
+    expect: {
+        do {
+            0, 0, null;
+        } while (console.log("PASS"));
+    }
+    expect_stdout: "PASS"
+}