fix corner case in loops & unused (#4076)

fixes #4075

lib/compress.js

@@ -4677,7 +4677,9 @@ merge(Compressor.prototype, {
                     sym = sym.expression.tail_node();
                 }
                 var def = sym.definition();
-                if (!def || def.id in in_use_ids) return;
+                if (!def) return;
+                if (def.scope !== self) return;
+                if (def.id in in_use_ids) return;
                 log(sym, "Dropping unused loop variable {name}");
                 var value = node.object.drop_side_effect_free(compressor);
                 if (!value) return in_list ? List.skip : make_node(AST_EmptyStatement, node);

test/compress/loops.js

@@ -963,3 +963,25 @@ issue_3634_2: {
     }
     expect_stdout: "1"
 }
+
+issue_4075: {
+    options = {
+        loops: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function() {
+            for (a in { PASS: 0 });
+        })()
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function() {
+            for (a in { PASS: 0 });
+        })()
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}