fix corner case in mangle (#4966)

fixes #4965

README.md

@@ -1341,3 +1341,16 @@ To allow for better optimizations, the compiler makes various assumptions:
   // Actual:   "FAIL"
   ```
   UglifyJS may modify the input which in turn may suppress those errors.
+- Earlier versions of JavaScript will throw `TypeError` with the following:
+  ```javascript
+  (function() {
+      {
+          const a = "foo";
+      }
+      {
+          const a = "bar";
+      }
+  })();
+  // TypeError: const 'a' has already been declared
+  ```
+  UglifyJS may modify the input which in turn may suppress those errors.

lib/scope.js

@@ -87,7 +87,7 @@ SymbolDef.prototype = {
         var def = scope.variables.get(name)
             || scope instanceof AST_Toplevel && scope.globals.get(name)
             || self.orig[0] instanceof AST_SymbolConst && find_if(function(def) {
-                return def.name == name && def !== self;
+                return def.name == name;
             }, scope.enclosed);
         if (def && def !== self) return def.redefined() || def;
     },
@@ -123,7 +123,6 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
 
     // pass 1: setup scope chaining and handle definitions
     var self = this;
-    var const_names = null;
     var defun = null;
     var exported = false;
     var next_def_id = 0;
@@ -202,7 +201,6 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         } else if (node instanceof AST_SymbolConst) {
             var def = scope.def_variable(node);
             def.defun = defun;
-            const_names.add(def.name, def);
             if (exported) def.exported = true;
         } else if (node instanceof AST_SymbolDefun) {
             var def = defun.def_function(node, tw.parent());
@@ -225,21 +223,13 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
 
         function walk_scope(descend) {
             node.init_vars(scope);
-            var save_names = const_names;
             var save_defun = defun;
             var save_scope = scope;
-            if (node instanceof AST_Scope) {
-                const_names = new Dictionary();
-                defun = node;
-            }
+            if (node instanceof AST_Scope) defun = node;
             scope = node;
             descend();
-            if (node instanceof AST_Scope) const_names.each(function(defs, name) {
-                if (defs.length > 1 && !node.variables.has(name)) push_uniq(node.enclosed, defs[0]);
-            });
             scope = save_scope;
             defun = save_defun;
-            const_names = save_names;
         }
 
         function entangle(defun, scope) {
@@ -680,9 +670,6 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options) {
             if (def.scope.parent_scope.find_variable(sym.name)) return false;
             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);
         def.references.forEach(reference);

test/compress/const.js

@@ -1539,6 +1539,7 @@ issue_4848: {
 issue_4954_1: {
     rename = true
     input: {
+        "use strict";
         (function() {
             {
                 const a = "foo";
@@ -1551,18 +1552,23 @@ issue_4954_1: {
         })();
     }
     expect: {
+        "use strict";
         (function() {
             {
                 const a = "foo";
                 console.log(a);
             }
             {
-                const a = "bar";
-                console.log(a);
+                const b = "bar";
+                console.log(b);
             }
         })();
     }
-    expect_stdout: true
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+    node_version: ">=4"
 }
 
 issue_4954_2: {
@@ -1610,11 +1616,79 @@ issue_4960: {
             {
                 const o = console.log("PASS");
             }
-            try {} catch (c) {
-                const o = console.log("FAIL");
+            try {} catch (o) {
+                const c = console.log("FAIL");
             }
         })();
     }
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_4965_1: {
+    mangle = {}
+    input: {
+        "use strict";
+        try {
+            c;
+        } catch (a) {
+            {
+                const a = 1;
+            }
+            {
+                const a = console.log(typeof c);
+            }
+        }
+    }
+    expect: {
+        "use strict";
+        try {
+            c;
+        } catch (t) {
+            {
+                const c = 1;
+            }
+            {
+                const t = console.log(typeof c);
+            }
+        }
+    }
+    expect_stdout: "undefined"
+    node_version: ">=4"
+}
+
+issue_4965_2: {
+    mangle = {}
+    input: {
+        "use strict";
+        try {
+            throw 1;
+        } catch (e) {
+            try {
+                {
+                    const e = 2;
+                }
+            } finally {
+                const e = 3;
+                console.log(typeof t);
+            }
+        }
+    }
+    expect: {
+        "use strict";
+        try {
+            throw 1;
+        } catch (o) {
+            try {
+                {
+                    const t = 2;
+                }
+            } finally {
+                const o = 3;
+                console.log(typeof t);
+            }
+        }
+    }
+    expect_stdout: "undefined"
+    node_version: ">=4"
+}