enable --module by default (#5738)

- fix corner case in `mangle`

lib/compress.js

@@ -10234,22 +10234,24 @@ Compressor.prototype.compress = function(node) {
     }
 
     function varify(self, compressor) {
-        return all(self.definitions, function(defn) {
+        if (all(self.definitions, function(defn) {
             return !defn.name.match_symbol(function(node) {
                 if (node instanceof AST_SymbolDeclaration) return !can_varify(compressor, node);
             }, true);
-        }) ? to_var(self, compressor.find_parent(AST_Scope)) : self;
+        })) return to_var(self, compressor.find_parent(AST_Scope));
     }
 
     OPT(AST_Const, function(self, compressor) {
         if (!compressor.option("varify")) return self;
+        var decl = varify(self, compressor);
+        if (decl) return decl;
         if (can_letify(self, compressor, 0)) return to_let(self);
-        return varify(self, compressor);
+        return self;
     });
 
     OPT(AST_Let, function(self, compressor) {
         if (!compressor.option("varify")) return self;
-        return varify(self, compressor);
+        return varify(self, compressor) || self;
     });
 
     function trim_optional_chain(node, compressor) {

lib/minify.js

@@ -82,14 +82,14 @@ function minify(files, options) {
             keep_fargs: false,
             keep_fnames: false,
             mangle: {},
-            module: false,
+            module: undefined,
             nameCache: null,
             output: {},
             parse: {},
             rename: undefined,
             sourceMap: false,
             timings: false,
-            toplevel: !!(options && !options["expression"] && options["module"]),
+            toplevel: undefined,
             v8: false,
             validate: false,
             warnings: false,
@@ -104,8 +104,10 @@ function minify(files, options) {
         if (options.ie) set_shorthand("ie", options, [ "compress", "mangle", "output", "rename" ]);
         if (options.keep_fargs) set_shorthand("keep_fargs", options, [ "compress", "mangle", "rename" ]);
         if (options.keep_fnames) set_shorthand("keep_fnames", options, [ "compress", "mangle", "rename" ]);
+        if (options.module === undefined && !options.ie) options.module = true;
         if (options.module) set_shorthand("module", options, [ "compress", "parse" ]);
-        if (options.toplevel) set_shorthand("toplevel", options, [ "compress", "mangle", "rename" ]);
+        if (options.toplevel === undefined && !options.expression && options.module) options.toplevel = true;
+        if (options.toplevel !== undefined) set_shorthand("toplevel", options, [ "compress", "mangle", "rename" ]);
         if (options.v8) set_shorthand("v8", options, [ "mangle", "output", "rename" ]);
         if (options.webkit) set_shorthand("webkit", options, [ "compress", "mangle", "output", "rename" ]);
         var quoted_props;

lib/scope.js

@@ -68,9 +68,7 @@ SymbolDef.prototype = {
         var cache = this.global && options.cache && options.cache.props;
         if (cache && cache.has(this.name)) {
             this.mangled_name = cache.get(this.name);
-        } else if (this.unmangleable(options)) {
-            names_in_use(this.scope, options).set(this.name, true);
-        } else {
+        } else if (!this.unmangleable(options)) {
             var def = this.redefined();
             if (def) {
                 this.mangled_name = def.mangled_name || def.name;
@@ -651,8 +649,12 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options) {
                 }, true);
             }
             var to_mangle = node.to_mangle = [];
-            node.variables.each(function(def) {
-                if (!defer_redef(def)) to_mangle.push(def);
+            node.variables.each(function(def, name) {
+                if (def.unmangleable(options)) {
+                    names_in_use(node, options).set(name, true);
+                } else if (!defer_redef(def)) {
+                    to_mangle.push(def);
+                }
             });
             descend();
             if (options.cache && node instanceof AST_Toplevel) {