a trailing comma in argument list is not an error (PY-4016)

python/src/com/jetbrains/python/parsing/FunctionParsing.java

@@ -157,6 +157,9 @@ public class FunctionParsing extends Parsing {
       }
       else {
         parameter.rollbackTo();
+        if (atToken(endToken)) {
+          break;
+        }
         PsiBuilder.Marker invalidElements = myBuilder.mark();
         while (!atToken(endToken) && !atToken(PyTokenTypes.LINE_BREAK) && !atToken(PyTokenTypes.COMMA) && !atToken(null)) {
           nextToken();

python/testData/psi/TrailingCommaInArgList.py

@@ -0,0 +1,2 @@
+def some_function(a,):
+    pass

python/testData/psi/TrailingCommaInArgList.txt

@@ -0,0 +1,16 @@
+PyFile:TrailingCommaInArgList.py
+  PyFunction('some_function')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('some_function')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PyNamedParameter('a')
+        PsiElement(Py:IDENTIFIER)('a')
+      PsiElement(Py:COMMA)(',')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyPassStatement
+        PsiElement(Py:PASS_KEYWORD)('pass')

python/testSrc/com/jetbrains/python/PythonParsingTest.java

@@ -271,6 +271,10 @@ public class PythonParsingTest extends ParsingTestCase {
     doTest();
   }
 
+  public void testTrailingCommaInArgList() {  // PY-4016
+    doTest();
+  }
+
   public void doTest() {
     doTest(LanguageLevel.PYTHON25);
   }