Support namedtuple as one of multiple superclasses (#2091)

Solve issue #1558 by delegate subtype checks on TupleType to fallback.

This does not solve #1485. I don't know why.

Author: elazarg

mypy/subtypes.py

@@ -163,20 +163,17 @@ def visit_callable_type(self, left: CallableType) -> bool:
     def visit_tuple_type(self, left: TupleType) -> bool:
         right = self.right
         if isinstance(right, Instance):
-            if is_named_instance(right, 'builtins.object'):
+            if is_named_instance(right, 'typing.Sized'):
                 return True
-            if is_named_instance(right, 'builtins.tuple'):
-                target_item_type = right.args[0]
-                return all(is_subtype(item, target_item_type)
-                           for item in left.items)
-            elif is_named_instance(right, 'typing.Sized'):
-                return True
-            elif (is_named_instance(right, 'typing.Iterable') or
+            elif (is_named_instance(right, 'builtins.tuple') or
+                  is_named_instance(right, 'typing.Iterable') or
                   is_named_instance(right, 'typing.Container') or
                   is_named_instance(right, 'typing.Sequence') or
                   is_named_instance(right, 'typing.Reversible')):
                 iter_type = right.args[0]
                 return all(is_subtype(li, iter_type) for li in left.items)
+            elif is_subtype(left.fallback, right, self.check_type_parameter):
+                return True
             return False
         elif isinstance(right, TupleType):
             if len(left.items) != len(right.items):

test-data/unit/check-namedtuple.test

@@ -322,4 +322,24 @@ reveal_type(X._field_types)  # E: Revealed type is 'builtins.dict[builtins.str,
 x = None  # type: X
 reveal_type(x._field_types)  # E: Revealed type is 'builtins.dict[builtins.str, Any]'
 
-[builtins fixtures/dict.pyi]
+[builtins fixtures/dict.pyi]
+
+[case testNamedTupleAndOtherSuperclass]
+from typing import NamedTuple
+
+class A: pass
+def f(x: A) -> None: pass
+
+class B(NamedTuple('B', []), A): pass
+f(B())
+x = None  # type: A
+x = B()
+
+# Sanity check: fail if baseclass does not match
+class C: pass
+def g(x: C) -> None: pass
+class D(NamedTuple('D', []), A): pass
+
+g(D())  # E: Argument 1 to "g" has incompatible type "D"; expected "C"
+y = None  # type: C
+y = D()  # E: Incompatible types in assignment (expression has type "D", variable has type "C")