fix corner case in unused (#4026)

fixes #4025

lib/compress.js

@@ -4457,13 +4457,18 @@ merge(Compressor.prototype, {
                     var sym = def.name.definition();
                     if (!drop_vars || sym.id in in_use_ids) {
                         if (def.value && indexOf_assign(sym, def) < 0) {
+                            var write_only = def.value.write_only;
                             var value = def.value.drop_side_effect_free(compressor);
-                            if (def.value !== value) def.value = value && make_sequence(def.value, [
-                                value,
-                                make_node(AST_Number, def.value, {
-                                    value: 0
-                                }),
-                            ]);
+                            if (def.value !== value) {
+                                def.value = value && make_sequence(def.value, [
+                                    value,
+                                    make_node(AST_Number, def.value, {
+                                        value: 0
+                                    }),
+                                ]);
+                            } else if (def.value.write_only !== write_only) {
+                                def.value.write_only = write_only;
+                            }
                         }
                         var old_def, var_defs = var_defs_by_id.get(sym.id);
                         if (!def.value) {

test/compress/drop-unused.js

@@ -2813,3 +2813,38 @@ issue_4017: {
     }
     expect_stdout: "undefined"
 }
+
+issue_4025: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        passes: 2,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 0, b = 0, c = 0, d = a++;
+        try {
+            var e = console.log(c), f = b;
+        } finally {
+            var d = b = 1, d = c + 1;
+            c = 0;
+        }
+        console.log(a, b, d);
+    }
+    expect: {
+        var d, c = 0;
+        try {
+            console.log(c);
+        } finally {
+            d = c + 1;
+            c = 0;
+        }
+        console.log(1, 1, d);
+    }
+    expect_stdout: [
+        "0",
+        "1 1 1",
+    ]
+}