Fix yield from semantics for AnyType

Revised version of someone else's PR that also should also
avoid a crash when the type isn't some sort of Instance. Avoids
cast using the now-possible `if not isinstance: return` construction.

Author: jhance

mypy/checker.py

@@ -1744,10 +1744,16 @@ def visit_call_expr(self, e: CallExpr) -> Type:
     def visit_yield_from_expr(self, e: YieldFromExpr) -> Type:
         # result = self.expr_checker.visit_yield_from_expr(e)
         result = self.accept(e.expr)
-        result_instance = cast(Instance, result)
-        if result_instance.type.fullname() == "asyncio.futures.Future":
+
+        # If the yield-from isn't typechecked (ie, its Any), don't typecheck
+        if isinstance(result, AnyType):
+            return AnyType()
+
+        if not isinstance(result, Instance):
+            self.msg.yield_from_invalid_operand_type(e.expr.accept(self), e)
+        elif result.type.fullname() == "asyncio.futures.Future":
             self.function_stack[-1].is_coroutine = True  # Set the function as coroutine
-            result = result_instance.args[0]  # Set the return type as the type inside
+            result = result.args[0]  # Set the return type as the type inside
         elif is_subtype(result, self.named_type('typing.Iterable')):
             # TODO
             # Check return type Iterator[Some]

mypy/test/data/check-statements.test

@@ -725,6 +725,12 @@ def g() -> Iterator[str]:
     yield "g2 more spam"
 [out]
 
+[case testYieldFromAny]
+from typing import Iterator
+def f(a):
+    b = yield from a
+    return b
+[out]
 
 -- With statement
 -- --------------