Remove use of join during semantic analysis

MROs may not be populated yet, so the join may crash. See #3316
for additional context. Fixes #3315.

Author: JukkaL

mypy/semanal.py

@@ -2048,9 +2048,12 @@ def build_namedtuple_typeinfo(self, name: str, items: List[str], types: List[Typ
         # Actual signature should return OrderedDict[str, Union[types]]
         ordereddictype = (self.named_type_or_none('builtins.dict', [strtype, AnyType()])
                           or self.object_type())
-        # 'builtins.tuple' has only one type parameter, the corresponding type argument
-        #  in the fallback instance is a join of all item types.
-        fallback = self.named_type('__builtins__.tuple', [join.join_type_list(types)])
+        # 'builtins.tuple' has only one type parameter.
+        #
+        # TODO: The corresponding type argument in the fallback instance should be a join of
+        #       all item types, but we can't do joins during this pass of semantic analysis
+        #       and we are using Any as a workaround.
+        fallback = self.named_type('__builtins__.tuple', [AnyType()])
         # Note: actual signature should accept an invariant version of Iterable[UnionType[types]].
         # but it can't be expressed. 'new' and 'len' should be callable types.
         iterable_type = self.named_type_or_none('typing.Iterable', [AnyType()])

test-data/unit/check-namedtuple.test

@@ -449,3 +449,13 @@ def f(x: a.X) -> None:
 [out]
 tmp/b.py:6: error: Revealed type is 'a.X'
 tmp/b.py:8: error: Revealed type is 'Tuple[Any, fallback=a.X]'
+
+[case testForwardReferenceInNamedTuple]
+from typing import NamedTuple
+
+class A(NamedTuple):
+    b: 'B'
+    x: int
+
+class B:
+    pass