Annotate 'yield' inside async functions as errors (PY-16094)

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

@@ -21,7 +21,7 @@ import com.jetbrains.python.codeInsight.dataflow.scope.ScopeUtil;
 import com.jetbrains.python.psi.*;
 
 /**
- * Highlights incorrect return statements: 'return' and 'yield' outside functions, returning values from generators.
+ * Highlights incorrect return statements: 'return' and 'yield' outside functions, 'yield' inside async functions.
  */
 public class ReturnAnnotator extends PyAnnotator {
   public void visitPyReturnStatement(final PyReturnStatement node) {
@@ -36,5 +36,8 @@ public class ReturnAnnotator extends PyAnnotator {
     if (!(owner instanceof PyFunction || owner instanceof PyLambdaExpression)) {
       getHolder().createErrorAnnotation(node, "'yield' outside of function");
     }
+    if (owner instanceof PyFunction && ((PyFunction)owner).isAsync()) {
+      getHolder().createErrorAnnotation(node, "'yield' inside async function");
+    }
   }
 }

python/testData/highlighting/yieldInsideAsyncDef.py

@@ -0,0 +1,5 @@
+async def foo(x):
+    await x
+    <error descr="'yield' inside async function">yield x</error>
+    <error descr="'yield' inside async function">yield from x</error>
+    return x

python/testSrc/com/jetbrains/python/PythonHighlightingTest.java

@@ -237,6 +237,10 @@ public class PythonHighlightingTest extends PyTestCase {
     doTest(LanguageLevel.PYTHON35, true, true);
   }
 
+  public void testYieldInsideAsyncDef() {
+    doTest(LanguageLevel.PYTHON35, false, false);
+  }
+
   // ---
   private void doTest(final LanguageLevel languageLevel, final boolean checkWarnings, final boolean checkInfos) {
     PythonLanguageLevelPusher.setForcedLanguageLevel(myFixture.getProject(), languageLevel);