improve keyword-related parser errors (#1941)

fixes #1937

lib/parse.js

@@ -801,17 +801,16 @@ function parse($TEXT, options) {
     };
 
     var statement = embed_tokens(function() {
-        var tmp;
         handle_regexp();
         switch (S.token.type) {
           case "string":
             if (S.in_directives) {
-                tmp = peek();
+                var token = peek();
                 if (S.token.raw.indexOf("\\") == -1
-                    && (tmp.nlb
+                    && (token.nlb
-                        || is_token(tmp, "eof")
+                        || is_token(token, "eof")
-                        || is_token(tmp, "punc", ";")
+                        || is_token(token, "punc", ";")
-                        || is_token(tmp, "punc", "}"))) {
+                        || is_token(token, "punc", "}"))) {
                     S.input.add_directive(S.token.value);
                 } else {
                     S.in_directives = false;
@@ -850,72 +849,97 @@ function parse($TEXT, options) {
             }
 
           case "keyword":
-            switch (tmp = S.token.value, next(), tmp) {
+            switch (S.token.value) {
               case "break":
+                next();
                 return break_cont(AST_Break);
 
               case "continue":
+                next();
                 return break_cont(AST_Continue);
 
               case "debugger":
+                next();
                 semicolon();
                 return new AST_Debugger();
 
               case "do":
+                next();
+                var body = in_loop(statement);
+                expect_token("keyword", "while");
+                var condition = parenthesised();
+                semicolon(true);
                 return new AST_Do({
-                    body      : in_loop(statement),
+                    body      : body,
-                    condition : (expect_token("keyword", "while"), tmp = parenthesised(), semicolon(true), tmp)
+                    condition : condition
                 });
 
               case "while":
+                next();
                 return new AST_While({
                     condition : parenthesised(),
                     body      : in_loop(statement)
                 });
 
               case "for":
+                next();
                 return for_();
 
               case "function":
+                next();
                 return function_(AST_Defun);
 
               case "if":
+                next();
                 return if_();
 
               case "return":
                 if (S.in_function == 0 && !options.bare_returns)
                     croak("'return' outside of function");
+                next();
+                var value = null;
+                if (is("punc", ";")) {
+                    next();
+                } else if (!can_insert_semicolon()) {
+                    value = expression(true);
+                    semicolon();
+                }
                 return new AST_Return({
-                    value: ( is("punc", ";")
+                    value: value
-                             ? (next(), null)
-                             : can_insert_semicolon()
-                             ? null
-                             : (tmp = expression(true), semicolon(), tmp) )
                 });
 
               case "switch":
+                next();
                 return new AST_Switch({
                     expression : parenthesised(),
                     body       : in_loop(switch_body_)
                 });
 
               case "throw":
+                next();
                 if (S.token.nlb)
                     croak("Illegal newline after 'throw'");
+                var value = expression(true);
+                semicolon();
                 return new AST_Throw({
-                    value: (tmp = expression(true), semicolon(), tmp)
+                    value: value
                 });
 
               case "try":
+                next();
                 return try_();
 
               case "var":
-                return tmp = var_(), semicolon(), tmp;
+                next();
+                var node = var_();
+                semicolon();
+                return node;
 
               case "with":
                 if (S.input.has_directive("use strict")) {
                     croak("Strict mode may not include a with statement");
                 }
+                next();
                 return new AST_With({
                     expression : parenthesised(),
                     body       : statement()

test/input/invalid/else.js

@@ -0,0 +1 @@
+if (0) else 1;

test/input/invalid/return.js

@@ -0,0 +1 @@
+return 42;

test/mocha/cli.js

@@ -469,6 +469,36 @@ describe("bin/uglifyjs", function () {
             done();
         });
     });
+    it("Should throw syntax error (else)", function(done) {
+        var command = uglifyjscmd + ' test/input/invalid/else.js';
+
+        exec(command, function (err, stdout, stderr) {
+            assert.ok(err);
+            assert.strictEqual(stdout, "");
+            assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
+                "Parse error at test/input/invalid/else.js:1,7",
+                "if (0) else 1;",
+                "       ^",
+                "ERROR: Unexpected token: keyword (else)"
+            ].join("\n"));
+            done();
+        });
+    });
+    it("Should throw syntax error (return)", function(done) {
+        var command = uglifyjscmd + ' test/input/invalid/return.js';
+
+        exec(command, function (err, stdout, stderr) {
+            assert.ok(err);
+            assert.strictEqual(stdout, "");
+            assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
+                "Parse error at test/input/invalid/return.js:1,0",
+                "return 42;",
+                "^",
+                "ERROR: 'return' outside of function"
+            ].join("\n"));
+            done();
+        });
+    });
     it("Should handle literal string as source map input", function(done) {
         var command = [
             uglifyjscmd,