PY-25784 Warn about use of star expressions in return and yield statements

False positive from "Redundant parentheses" inspection has been fixed
earlier for PY-20324.

python/src/com/jetbrains/python/validation/StarAnnotator.java

@@ -15,8 +15,13 @@
  */
 package com.jetbrains.python.validation;
 
+import com.intellij.psi.PsiElement;
 import com.jetbrains.python.codeInsight.functionTypeComments.psi.PyParameterTypeList;
+import com.jetbrains.python.psi.PyReturnStatement;
 import com.jetbrains.python.psi.PyStarExpression;
+import com.jetbrains.python.psi.PyTupleExpression;
+import com.jetbrains.python.psi.PyYieldExpression;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * @author yole
@@ -25,8 +30,21 @@ public class StarAnnotator extends PyAnnotator {
   @Override
   public void visitPyStarExpression(PyStarExpression node) {
     super.visitPyStarExpression(node);
-    if (!node.isAssignmentTarget() && !node.isUnpacking() && !(node.getParent() instanceof PyParameterTypeList)) {
+    if (!node.isAssignmentTarget() && !allowedUnpacking(node) && !(node.getParent() instanceof PyParameterTypeList)) {
       getHolder().createErrorAnnotation(node, "Can't use starred expression here");
     }
   }
+
+  private static boolean allowedUnpacking(@NotNull PyStarExpression starExpression) {
+    if (!starExpression.isUnpacking()) {
+      return false;
+    }
+
+    final PsiElement parent = starExpression.getParent();
+    if (parent instanceof PyTupleExpression && (parent.getParent() instanceof PyReturnStatement ||
+                                                parent.getParent() instanceof PyYieldExpression)) {
+      return false;
+    }
+    return true;
+  }
 }

python/testData/highlighting/unpackingStar.py

@@ -7,4 +7,13 @@ if <error descr="Can't use starred expression here">*x</error>:
     pass
 
 1 + (<error descr="Can't use starred expression here">*x</error>)
-1 + (*x,)
+1 + (*x,)
+
+
+def f(x):
+    return x, <error descr="Can't use starred expression here">*x</error>
+
+
+def g(x):
+    yield from x, <error descr="Can't use starred expression here">*x</error>
+    yield x, <error descr="Can't use starred expression here">*x</error>