fix corner case in ie8 (#4532)

fixes #4531

lib/scope.js

@@ -311,8 +311,9 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         }
         if (node instanceof AST_SymbolLambda) {
             var def = node.thedef;
-            redefine(node, node.scope.parent_scope.resolve());
-            if (typeof node.thedef.init !== "undefined") {
+            if (!redefine(node, node.scope.parent_scope.resolve())) {
+                delete def.defun;
+            } else if (typeof node.thedef.init !== "undefined") {
                 node.thedef.init = false;
             } else if (def.init) {
                 node.thedef.init = def.init;
@@ -332,7 +333,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         var old_def = node.thedef;
         if (!all(old_def.orig, function(sym) {
             return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet);
-        })) return;
+        })) return false;
         var new_def = scope.find_variable(name);
         if (new_def) {
             var redef = new_def.redefined();
@@ -352,6 +353,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
             node.reference(options);
         });
         if (new_def.undeclared) self.variables.set(name, new_def);
+        return true;
     }
 });
 

test/compress/let.js

@@ -1282,3 +1282,60 @@ issue_4438: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_4531_1: {
+    mangle = {
+        ie8: true,
+        toplevel: true,
+    }
+    input: {
+        "use strict";
+        var a;
+        console.log(function a() {
+            let a;
+            var b;
+        }());
+    }
+    expect: {
+        "use strict";
+        var o;
+        console.log(function o() {
+            let o;
+            var t;
+        }());
+    }
+    expect_stdout: "undefined"
+    node_version: ">=4"
+}
+
+issue_4531_2: {
+    options = {
+        evaluate: true,
+        ie8: true,
+        toplevel: true,
+    }
+    mangle = {
+        ie8: true,
+        toplevel: true,
+    }
+    input: {
+        "use strict";
+        var a = console;
+        console.log(typeof a, function a() {
+            let { [console]: a } = 0 && a;
+            var b = console;
+            while (!b);
+        }());
+    }
+    expect: {
+        "use strict";
+        var o = console;
+        console.log(typeof o, function o() {
+            let { [console]: o } = 0;
+            var e = console;
+            while (!e);
+        }());
+    }
+    expect_stdout: "object undefined"
+    node_version: ">=6"
+}