Fixed false negative in parsing incomplete 'yield from' expression (PY-6733)

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

@@ -533,10 +533,18 @@ public class ExpressionParsing extends Parsing {
       myBuilder.advanceLexer();
       if (myBuilder.getTokenType() == PyTokenTypes.FROM_KEYWORD) {
         myBuilder.advanceLexer();
+        final boolean parsed = parseTupleExpression(false, isTargetExpression, false);
+        if (!parsed) {
+          myBuilder.error(message("PARSE.expected.expression"));
+        }
+        yieldExpr.done(PyElementTypes.YIELD_EXPRESSION);
+        return parsed;
+      }
+      else {
+        parseTupleExpression(false, isTargetExpression, false);
+        yieldExpr.done(PyElementTypes.YIELD_EXPRESSION);
+        return true;
       }
-      parseTupleExpression(false, isTargetExpression, false);
-      yieldExpr.done(PyElementTypes.YIELD_EXPRESSION);
-      return true;
     }
     else {
       return parseTupleExpression(false, isTargetExpression, false);

python/testData/psi/YieldFromNoExpr.py

@@ -0,0 +1,2 @@
+def f():
+    yield from

python/testData/psi/YieldFromNoExpr.txt

@@ -0,0 +1,18 @@
+PyFile:YieldFromNoExpr.py
+  PyFunction('f')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('f')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyExpressionStatement
+        PyYieldExpression
+          PsiElement(Py:YIELD_KEYWORD)('yield')
+          PsiWhiteSpace(' ')
+          PsiElement(Py:FROM_KEYWORD)('from')
+          PsiErrorElement:expression expected
+            <empty list>

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

@@ -323,6 +323,11 @@ public class PythonParsingTest extends ParsingTestCase {
     doTest(LanguageLevel.PYTHON33);
   }
 
+  // PY-6733
+  public void testYieldFromNoExpr() {
+    doTest(LanguageLevel.PYTHON33);
+  }
+
   public void doTest() {
     doTest(LanguageLevel.PYTHON25);
   }