@Nullable
  private PyType getYieldStatementType(@NotNull final TypeEvalContext context) {
    Ref<PyType> elementType = null;
    final PyBuiltinCache cache = PyBuiltinCache.getInstance(this);
    final PyClass listClass = cache.getClass("list");
    final PyStatementList statements = getStatementList();
    final Set<PyType> types = new LinkedHashSet<PyType>();
    if (statements != null && listClass != null) {
      statements.accept(
          new PyRecursiveElementVisitor() {
            @Override
            public void visitPyYieldExpression(PyYieldExpression node) {
              final PyType type = context.getType(node);
              if (node.isDelegating() && type instanceof PyCollectionType) {
                final PyCollectionType collectionType = (PyCollectionType) type;
                types.add(collectionType.getElementType(context));
              } else {
                types.add(type);
              }
            }

            @Override
            public void visitPyFunction(PyFunction node) {
              // Ignore nested functions
            }
          });
      final int n = types.size();
      if (n == 1) {
        elementType = Ref.create(types.iterator().next());
      } else if (n > 0) {
        elementType = Ref.create(PyUnionType.union(types));
      }
    }
    if (elementType != null) {
      final PyClass generator = cache.getClass(PyNames.FAKE_GENERATOR);
      if (generator != null) {
        return new PyCollectionTypeImpl(generator, false, elementType.get());
      }
    }
    return null;
  }
 @Nullable
 private PyType createCoroutineType(@Nullable PyType returnType) {
   final PyBuiltinCache cache = PyBuiltinCache.getInstance(this);
   if (returnType instanceof PyClassLikeType
       && PyNames.FAKE_COROUTINE.equals(((PyClassLikeType) returnType).getClassQName())) {
     return returnType;
   }
   final PyClass generator = cache.getClass(PyNames.FAKE_COROUTINE);
   return generator != null
       ? new PyCollectionTypeImpl(generator, false, Collections.singletonList(returnType))
       : null;
 }
  @Nullable
  private Ref<? extends PyType> getYieldStatementType(@NotNull final TypeEvalContext context) {
    Ref<PyType> elementType = null;
    final PyBuiltinCache cache = PyBuiltinCache.getInstance(this);
    final PyStatementList statements = getStatementList();
    final Set<PyType> types = new LinkedHashSet<>();
    statements.accept(
        new PyRecursiveElementVisitor() {
          @Override
          public void visitPyYieldExpression(PyYieldExpression node) {
            final PyExpression expr = node.getExpression();
            final PyType type = expr != null ? context.getType(expr) : null;
            if (node.isDelegating() && type instanceof PyCollectionType) {
              final PyCollectionType collectionType = (PyCollectionType) type;
              // TODO: Select the parameter types that matches T in Iterable[T]
              final List<PyType> elementTypes = collectionType.getElementTypes(context);
              types.add(elementTypes.isEmpty() ? null : elementTypes.get(0));
            } else {
              types.add(type);
            }
          }

          @Override
          public void visitPyFunction(PyFunction node) {
            // Ignore nested functions
          }
        });
    final int n = types.size();
    if (n == 1) {
      elementType = Ref.create(types.iterator().next());
    } else if (n > 0) {
      elementType = Ref.create(PyUnionType.union(types));
    }
    if (elementType != null) {
      final PyClass generator = cache.getClass(PyNames.FAKE_GENERATOR);
      if (generator != null) {
        final List<PyType> parameters =
            Arrays.asList(elementType.get(), null, getReturnStatementType(context));
        return Ref.create(new PyCollectionTypeImpl(generator, false, parameters));
      }
    }
    if (!types.isEmpty()) {
      return Ref.create(null);
    }
    return null;
  }