Fix crash on incompatible redefinition in named tuple

mypy/checker.py

@@ -1440,8 +1440,11 @@ def lvalue_type_from_base(self, expr_node: Var,
 
             if base_type:
                 if not has_no_typevars(base_type):
-                    # TODO: Handle TupleType, don't cast
-                    instance = cast(Instance, self.scope.active_self_type())
+                    self_type = self.scope.active_self_type()
+                    if isinstance(self_type, TupleType):
+                        instance = self_type.fallback
+                    else:
+                        instance = self_type
                     itype = map_instance_to_supertype(instance, base)
                     base_type = expand_type_by_instance(base_type, itype)
 

test-data/unit/check-namedtuple.test

@@ -412,6 +412,12 @@ b = B._make([''])  # type: B
 
 [builtins fixtures/list.pyi]
 
+[case testNamedTupleIncompatibleRedefinition]
+from typing import NamedTuple
+class Crash(NamedTuple):
+    count: int  # E: Incompatible types in assignment (expression has type "int", base class "tuple" defined the type as Callable[[Tuple[Any, ...], Any], int])
+[builtins fixtures/tuple.pyi]
+
 [case testNamedTupleInClassNamespace]
 # https://github.com/python/mypy/pull/2553#issuecomment-266474341
 from typing import NamedTuple

test-data/unit/fixtures/tuple.pyi

@@ -1,6 +1,6 @@
 # Builtins stub used in tuple-related test cases.
 
-from typing import Iterable, Iterator, TypeVar, Generic, Sequence
+from typing import Iterable, Iterator, TypeVar, Generic, Sequence, Any
 
 Tco = TypeVar('Tco', covariant=True)
 
@@ -13,6 +13,7 @@ class type:
 class tuple(Sequence[Tco], Generic[Tco]):
     def __iter__(self) -> Iterator[Tco]: pass
     def __getitem__(self, x: int) -> Tco: pass
+    def count(self, obj: Any) -> int: pass
 class function: pass
 
 # We need int and slice for indexing tuples.