fix corner case in loops & unused (#4092)

fixes #4091

lib/compress.js

@@ -4642,16 +4642,7 @@ merge(Compressor.prototype, {
                         body: make_sequence(node, side_effects)
                     }));
                 }
-                switch (body.length) {
-                  case 0:
-                    return in_list ? List.skip : make_node(AST_EmptyStatement, node);
-                  case 1:
-                    return body[0];
-                  default:
-                    return in_list ? List.splice(body) : make_node(AST_BlockStatement, node, {
-                        body: body
-                    });
-                }
+                return insert_statements(body, node, in_list);
             }
             if (node instanceof AST_LabeledStatement && node.body instanceof AST_For) {
                 // Certain combination of unused name + side effect leads to invalid AST:
@@ -4714,12 +4705,19 @@ merge(Compressor.prototype, {
                 if (def.id in in_use_ids) return;
                 if (def.scope !== self && member(def, self.enclosed)) return;
                 log(sym, "Dropping unused loop variable {name}");
+                if (for_ins[def.id] === node) delete for_ins[def.id];
+                var body = [];
                 var value = node.object.drop_side_effect_free(compressor);
-                if (!value) return in_list ? List.skip : make_node(AST_EmptyStatement, node);
+                if (value) {
                     AST_Node.warn("Side effects in object of for-in loop [{file}:{line},{col}]", value.start);
-                return make_node(AST_SimpleStatement, node, {
+                    body.push(make_node(AST_SimpleStatement, node, {
                         body: value
-                });
+                    }));
+                }
+                if (node.init instanceof AST_Definitions && def.orig[0] instanceof AST_SymbolCatch) {
+                    body.push(node.init);
+                }
+                return insert_statements(body, node, in_list);
             } else if (node instanceof AST_Sequence) {
                 if (node.expressions.length == 1) return node.expressions[0];
             }
@@ -4752,6 +4750,19 @@ merge(Compressor.prototype, {
             };
         }
 
+        function insert_statements(body, orig, in_list) {
+            switch (body.length) {
+              case 0:
+                return in_list ? List.skip : make_node(AST_EmptyStatement, orig);
+              case 1:
+                return body[0];
+              default:
+                return in_list ? List.splice(body) : make_node(AST_BlockStatement, orig, {
+                    body: body
+                });
+            }
+        }
+
         function track_assigns(def, node) {
             if (def.scope !== self) return false;
             if (!def.fixed || !node.fixed) assign_in_use[def.id] = false;

test/compress/loops.js

@@ -1037,3 +1037,54 @@ issue_4084: {
     }
     expect_stdout: "undefined"
 }
+
+issue_4091_1: {
+    options = {
+        loops: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        try {
+            throw "FAIL";
+        } catch (e) {
+            for (var e in 42);
+        }
+        console.log(e && e);
+    }
+    expect: {
+        try {
+            throw "FAIL";
+        } catch (e) {
+            var e;
+        }
+        console.log(e && e);
+    }
+    expect_stdout: "undefined"
+}
+
+issue_4091_2: {
+    options = {
+        loops: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        try {
+            throw "FAIL";
+        } catch (e) {
+            for (e in 42);
+            var e;
+        }
+        console.log(e && e);
+    }
+    expect: {
+        try {
+            throw "FAIL";
+        } catch (e) {
+            var e;
+        }
+        console.log(e && e);
+    }
+    expect_stdout: "undefined"
+}