fix corner cases in loops & unused (#4995)

fixes #4994

lib/compress.js

@@ -6042,27 +6042,16 @@ merge(Compressor.prototype, {
             function compose(child, level, find) {
                 var parent = compressor.parent(level);
                 if (!parent) return find;
-                if (parent instanceof AST_DestructuredKeyVal) {
-                    var destructured = compressor.parent(level + 1);
-                    if (parent.key === child) {
-                        var fn = compressor.parent(level + 2);
-                        if (fn instanceof AST_Lambda) {
-                            return compose(fn, level + 3, fn.argnames.indexOf(destructured) >= 0 ? function(name) {
-                                var def = find(name);
-                                if (def) return def;
-                                def = fn.variables.get(name);
-                                if (def) {
-                                    var sym = def.orig[0];
-                                    if (sym instanceof AST_SymbolFunarg || sym instanceof AST_SymbolLambda) return def;
-                                }
-                            } : function(name) {
-                                return find(name) || fn.variables.get(name);
-                            });
-                        }
+                var in_arg = parent instanceof AST_Lambda && member(child, parent.argnames);
+                return compose(parent, level + 1, in_arg ? function(name) {
+                    var def = find(name);
+                    if (def) return def;
+                    def = parent.variables.get(name);
+                    if (def) {
+                        var sym = def.orig[0];
+                        if (sym instanceof AST_SymbolFunarg || sym instanceof AST_SymbolLambda) return def;
                     }
-                    return compose(destructured, level + 2, find);
-                }
-                return compose(parent, level + 1, parent.variables ? function(name) {
+                } : parent.variables ? function(name) {
                     return find(name) || parent.variables.get(name);
                 } : find);
             }

test/compress/default-values.js

@@ -1726,3 +1726,28 @@ issue_4916: {
     expect_stdout: "undefined"
     node_version: ">=6"
 }
+
+issue_4994: {
+    options = {
+        loops: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function(b = function() {
+            for (a in { PASS: 42 });
+        }()) {
+            var a;
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function(b = function() {
+            for (a in { PASS: 42 });
+        }()) {})();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/compress/destructured.js

@@ -2592,3 +2592,36 @@ issue_4608_2: {
     expect_stdout: "f"
     node_version: ">=6"
 }
+
+issue_4994: {
+    options = {
+        loops: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function([
+            {
+                [function() {
+                    for (a in { PASS: null });
+                }()]: b,
+            },
+        ]) {
+            var a;
+        })([ 42 ]);
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function([
+            {
+                [function() {
+                    for (a in { PASS: null });
+                }()]: b,
+            },
+        ]) {})([ 42 ]);
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}