fix corner case in mangle (#4961)

fixes #4960

lib/scope.js

@@ -70,9 +70,7 @@ SymbolDef.prototype = {
         } else if (!this.mangled_name && !this.unmangleable(options)) {
             var def = this.redefined();
             if (def) {
-                var name = def.mangled_name || def.name;
-                this.mangled_name = name;
-                names_in_use(this.scope, options)[name] = true;
+                this.mangled_name = def.mangled_name || def.name;
             } else {
                 this.mangled_name = next_mangled_name(this, options);
             }
@@ -683,6 +681,7 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options) {
             redef = scope.def_variable(sym);
             scope.to_mangle.push(redef);
         } else if (redef.mangled_name) {
+            names_in_use(def.scope, options)[redef.mangled_name] = true;
             return false;
         }
         redefined.push(def);

test/compress/const.js

@@ -1588,3 +1588,33 @@ issue_4954_2: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_4960: {
+    mangle = {}
+    input: {
+        "use strict";
+        var a;
+        (function() {
+            {
+                const a = console.log("PASS");
+            }
+            try {} catch (e) {
+                const a = console.log("FAIL");
+            }
+        })();
+    }
+    expect: {
+        "use strict";
+        var a;
+        (function() {
+            {
+                const o = console.log("PASS");
+            }
+            try {} catch (c) {
+                const o = console.log("FAIL");
+            }
+        })();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}