Fix yield from TupleType. (#6902)

Fixes #4444 by removing special-casing for instance types.
Now `yield from <tuple>` works as expected.

Author: brandtbucher

mypy/checker.py

@@ -2442,15 +2442,14 @@ def split_around_star(self, items: List[T], star_index: int,
     def type_is_iterable(self, type: Type) -> bool:
         if isinstance(type, CallableType) and type.is_type_obj():
             type = type.fallback
-        return (is_subtype(type, self.named_generic_type('typing.Iterable',
-                                                         [AnyType(TypeOfAny.special_form)])) and
-                isinstance(type, Instance))
+        return is_subtype(type, self.named_generic_type('typing.Iterable',
+                                                        [AnyType(TypeOfAny.special_form)]))
 
     def check_multi_assignment_from_iterable(self, lvalues: List[Lvalue], rvalue_type: Type,
                                              context: Context,
                                              infer_lvalue_type: bool = True) -> None:
-        if self.type_is_iterable(rvalue_type):
-            item_type = self.iterable_item_type(cast(Instance, rvalue_type))
+        if self.type_is_iterable(rvalue_type) and isinstance(rvalue_type, Instance):
+            item_type = self.iterable_item_type(rvalue_type)
             for lv in lvalues:
                 if isinstance(lv, StarExpr):
                     items_type = self.named_generic_type('builtins.list', [item_type])

test-data/unit/check-expressions.test

@@ -1694,6 +1694,16 @@ def g() -> Iterator[int]:
     a = yield from f()
 [out]
 
+[case testYieldFromTupleExpression]
+from typing import Generator
+def g() -> Generator[int, None, None]:
+    x = yield from ()  # E: Function does not return a value
+    x = yield from (0, 1, 2)  # E: Function does not return a value
+    x = yield from (0, "ERROR")  # E: Incompatible types in "yield from" (actual type "object", expected type "int") \
+                                 # E: Function does not return a value
+    x = yield from ("ERROR",)  # E: Incompatible types in "yield from" (actual type "str", expected type "int") \
+                               # E: Function does not return a value
+[builtins fixtures/tuple.pyi]
 
 -- dict(...)
 -- ---------

test-data/unit/check-statements.test

@@ -1250,6 +1250,15 @@ def g() -> Generator[int, str, float]:
     reveal_type(r)  # E: Revealed type is 'builtins.str*'
     return 3.14
 
+[case testYieldFromTupleStatement]
+from typing import Generator
+def g() -> Generator[int, None, None]:
+    yield from ()
+    yield from (0, 1, 2)
+    yield from (0, "ERROR")  # E: Incompatible types in "yield from" (actual type "object", expected type "int")
+    yield from ("ERROR",)  # E: Incompatible types in "yield from" (actual type "str", expected type "int")
+[builtins fixtures/tuple.pyi]
+
 -- With statement
 -- --------------