fix corner case in ie8 & mangle (#4196)

fixes #4195

lib/scope.js

@@ -68,8 +68,8 @@ SymbolDef.prototype = {
         if (this.global && cache && cache.has(this.name)) {
             this.mangled_name = cache.get(this.name);
         } else if (!this.mangled_name && !this.unmangleable(options)) {
-            var def;
-            if (def = this.redefined()) {
+            var def = this.redefined();
+            if (def) {
                 this.mangled_name = def.mangled_name || def.name;
             } else {
                 this.mangled_name = next_mangled_name(this.scope, options, this);
@@ -84,8 +84,8 @@ SymbolDef.prototype = {
         if (!scope) return;
         var def = scope.variables.get(this.name);
         if (!def && scope instanceof AST_Toplevel) def = scope.globals.get(this.name);
-        if (def === this) return;
-        return def;
+        if (!def || def === this) return;
+        return def.redefined() || def;
     },
     unmangleable: function(options) {
         return this.global && !options.toplevel
@@ -274,8 +274,8 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         var old_def = node.thedef;
         var new_def = scope.find_variable(name);
         if (new_def) {
-            var redef;
-            while (redef = new_def.redefined()) new_def = redef;
+            var redef = new_def.redefined();
+            if (redef) new_def = redef;
         } else {
             new_def = self.globals.get(name);
         }

test/compress/const.js

@@ -747,3 +747,32 @@ issue_4193: {
     }
     expect_stdout: true
 }
+
+issue_4195: {
+    mangle = {
+        ie8: true,
+    }
+    input: {
+        console.log(function f(a) {
+            (function a() {
+                {
+                    const b = f, a = 0;
+                    b;
+                }
+            })();
+            a && f;
+        }());
+    }
+    expect: {
+        console.log(function f(o) {
+            (function o() {
+                {
+                    const n = f, o = 0;
+                    n;
+                }
+            })();
+            o && f;
+        }());
+    }
+    expect_stdout: "undefined"
+}