Fix newline handling after yield

YieldExpressions can only be defined as:
 * `yield`
 * `yield` [no nlb] AssignmentExpression
 * `yield` [no nlb] `*` AssignmentExpression

lib/parse.js

@@ -1174,17 +1174,23 @@ function parse($TEXT, options) {
         var has_expression = true;
         var tmp;
 
-        // Get expression behind yield, default to value `undefined` stored as `null` in ast
-        // Expression must start on same line, yield* has a mandatory expression
-        if (is("operator", "*")) {
-            star = true;
-            next();
-            if (S.token.nlb) {
-                unexpected(S.prev);
-            }
-        } else if (can_insert_semicolon() ||
+        // Attempt to get expression or star (and then the mandatory expression)
+        // behind yield on the same line.
+        //
+        // If nothing follows on the same line of the yieldExpression,
+        // it should default to the value `undefined` for yield to return.
+        // In that case, the `undefined` stored as `null` in ast.
+        //
+        // Note 1: It isn't allowed for yield* to close without an expression
+        // Note 2: If there is a nlb between yield and star, it is interpret as
+        //         yield <explicit undefined> <inserted automatic semicolon> *
+        if (can_insert_semicolon() ||
             (is("punc") && PUNC_AFTER_EXPRESSION(S.token.value))) {
             has_expression = false;
+
+        } else if (is("operator", "*")) {
+            star = true;
+            next();
         }
 
         return new AST_Yield({

test/mocha/yield.js

@@ -20,8 +20,20 @@ describe("Yield", function() {
         assert.throws(test, expect);
     });
 
-    it("Should not allow yield* followed by a newline in generators", function() {
-        var js = "function* test() {yield*\n123;}";
+    it("Should not allow yield* followed by a semicolon in generators", function() {
+        var js = "function* test() {yield*\n;}";
+        var test = function() {
+            UglifyJS.parse(js);
+        }
+        var expect = function(e) {
+            return e instanceof UglifyJS.JS_Parse_Error &&
+                e.message === "Unexpected token: punc (;)";
+        }
+        assert.throws(test, expect);
+    });
+
+    it("Should not allow yield with next token star on next line", function() {
+        var js = "function* test() {yield\n*123;}";
         var test = function() {
             UglifyJS.parse(js);
         }