Update the overlapping check for tuples to account for NamedTuples (python#18564)

Fixes python#18562.

Fixes python#6623.

Fixes python#18520. (Only incidentally
and I'm not exactly sure why.)

I was investigating what input mypy thought satisfied both overloads and
I found that mypy is missing a check on the fallback type.

Author: A5rocks

mypy/meet.py

@@ -50,6 +50,7 @@
     find_unpack_in_list,
     get_proper_type,
     get_proper_types,
+    is_named_instance,
     split_with_prefix_and_suffix,
 )
 
@@ -645,7 +646,16 @@ def are_tuples_overlapping(
 
     if len(left.items) != len(right.items):
         return False
-    return all(is_overlapping(l, r) for l, r in zip(left.items, right.items))
+    if not all(is_overlapping(l, r) for l, r in zip(left.items, right.items)):
+        return False
+
+    # Check that the tuples aren't from e.g. different NamedTuples.
+    if is_named_instance(right.partial_fallback, "builtins.tuple") or is_named_instance(
+        left.partial_fallback, "builtins.tuple"
+    ):
+        return True
+    else:
+        return is_overlapping(left.partial_fallback, right.partial_fallback)
 
 
 def expand_tuple_if_possible(tup: TupleType, target: int) -> TupleType:

test-data/unit/check-namedtuple.test

@@ -1474,3 +1474,35 @@ def main(n: NT[T]) -> None:
 
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-namedtuple.pyi]
+
+[case testNamedTupleOverlappingCheck]
+from typing import overload, NamedTuple, Union
+
+class AKey(NamedTuple):
+    k: str
+
+class A(NamedTuple):
+    key: AKey
+
+
+class BKey(NamedTuple):
+    k: str
+
+class B(NamedTuple):
+    key: BKey
+
+@overload
+def f(arg: A) -> A: ...
+@overload
+def f(arg: B) -> B: ...
+def f(arg: Union[A, B]) -> Union[A, B]: ...
+
+def g(x: Union[A, B, str]) -> Union[A, B, str]:
+    if isinstance(x, str):
+        return x
+    else:
+        reveal_type(x)  # N: Revealed type is "Union[Tuple[Tuple[builtins.str, fallback=__main__.AKey], fallback=__main__.A], Tuple[Tuple[builtins.str, fallback=__main__.BKey], fallback=__main__.B]]"
+        return x._replace()
+
+# no errors should be raised above.
+[builtins fixtures/tuple.pyi]