fix corner case in unused (#3517)

fixes #3515

lib/compress.js

@@ -3737,7 +3737,9 @@ merge(Compressor.prototype, {
                         def.value = null;
                         head.push(def);
                     } else {
-                        var value = def.value && def.value.drop_side_effect_free(compressor);
+                        var value = def.value
+                            && (sym.references.length != 1 || !sym.replaced)
+                            && def.value.drop_side_effect_free(compressor);
                         if (value) {
                             AST_Node.warn("Side effects in initialization of unused variable {name} [{file}:{line},{col}]", template(def.name));
                             side_effects.push(value);

test/compress/drop-unused.js

@@ -2102,3 +2102,28 @@ issue_3497: {
     }
     expect_stdout: "undefined"
 }
+
+issue_3515: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var c = 0;
+        (function() {
+            this[c++] = 0;
+            var expr20 = !0;
+            for (var key20 in expr20);
+        })();
+        console.log(c);
+    }
+    expect: {
+        var c = 0;
+        (function() {
+            for (var key20 in !(this[c++] = 0));
+        })();
+        console.log(c);
+    }
+    expect_stdout: "1"
+}