complain about missing default value of a function argument (PY-3253)

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

@@ -148,7 +148,9 @@ public class FunctionParsing extends Parsing {
           annotationMarker.done(PyElementTypes.ANNOTATION);
         }
         if (!isStarParameter && matchToken(PyTokenTypes.EQ)) {
-          getExpressionParser().parseSingleExpression(false);
+          if (!getExpressionParser().parseSingleExpression(false)) {
+            myBuilder.error(message("PARSE.expected.expression"));
+          }
         }
         parameter.done(PyElementTypes.NAMED_PARAMETER);
       }

python/testData/psi/MissingDefaultValue.py

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

python/testData/psi/MissingDefaultValue.txt

@@ -0,0 +1,18 @@
+PyFile:MissingDefaultValue.py
+  PyFunction('fa')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('fa')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PyNamedParameter('a')
+        PsiElement(Py:IDENTIFIER)('a')
+        PsiElement(Py:EQ)('=')
+        PsiErrorElement:expression expected
+          <empty list>
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n   ')
+    PyStatementList
+      PyPassStatement
+        PsiElement(Py:PASS_KEYWORD)('pass')

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

@@ -263,6 +263,10 @@ public class PythonParsingTest extends ParsingTestCase {
     doTest();
   }
 
+  public void testMissingDefaultValue() {  // PY-3253
+    doTest();
+  }
+
   public void doTest() {
     doTest(LanguageLevel.PYTHON25);
   }