fix corner case in unused (#3674)

fixes #3673

lib/compress.js

@@ -4196,7 +4196,7 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_Assign) {
                     if (node.write_only === "p" && node.right.may_throw_on_access(compressor)) return;
                     var right = get_rhs(node);
-                    if (init && node.write_only && node_def.scope === self && !right.has_side_effects(compressor)) {
+                    if (init && node.write_only === true && node_def.scope === self && !right.has_side_effects(compressor)) {
                         initializations.add(node_def.id, right);
                     } else {
                         right.walk(tw);

test/compress/drop-unused.js

@@ -2355,3 +2355,23 @@ issue_3664: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3673: {
+    options = {
+        pure_getters: "strict",
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        (a = [ a ]).p = 42;
+        console.log("PASS");
+    }
+    expect: {
+        var a;
+        (a = [ a ]).p = 42;
+        console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}