Fix union of generators having no return type

mypy/checkexpr.py

@@ -5095,7 +5095,11 @@ def visit_yield_from_expr(self, e: YieldFromExpr, allow_none_return: bool = Fals
 
         # Determine the type of the entire yield from expression.
         iter_type = get_proper_type(iter_type)
-        if isinstance(iter_type, Instance) and iter_type.type.fullname == "typing.Generator":
+        if (
+            isinstance(iter_type, Instance)
+            and iter_type.type.fullname == "typing.Generator"
+            or isinstance(iter_type, UnionType)
+        ):
             expr_type = self.chk.get_generator_return_type(iter_type, False)
         else:
             # Non-Generators don't return anything from `yield from` expressions.

test-data/unit/check-statements.test

@@ -2195,6 +2195,32 @@ class B: pass
 def foo(x: int) -> Union[Generator[A, None, None], Generator[B, None, None]]:
     yield x  # E: Incompatible types in "yield" (actual type "int", expected type "Union[A, B]")
 
+[case testReturnTypeOfGeneratorUnion]
+from typing import Generator, Union
+
+class T: pass
+
+def foo(arg: Union[Generator[int, None, T], Generator[str, None, T]]) -> Generator[Union[int, str], None, T]:
+    return (yield from arg)
+
+[case testYieldFromError]
+from typing import Generator, Union
+
+class T: pass
+class R: pass
+
+def foo(arg: Union[T, R]) -> Generator[Union[int, str], None, T]:
+    return (yield from arg) # E: "yield from" cannot be applied to "Union[T, R]"
+
+[case testUnionOfGeneratorWithIterableStr]
+from typing import Generator, Union, Iterable
+
+class T: pass
+
+def foo(arg: Union[Generator[int, None, T], Iterable[str]]) -> Generator[Union[int, str], None, T]:
+    return (yield from arg)
+[builtins fixtures/tuple.pyi]
+
 [case testNoCrashOnStarRightHandSide]
 x = *(1, 2, 3)  # E: Can use starred expression only as assignment target
 [builtins fixtures/tuple.pyi]