typing.NamedTuple collides based on field order and type, doesn't consider class name or field names

[neilvyas]

I expect this code to typecheck, since A and B are nominally different. Note that changing the field type or adding additional fields results in A and B no longer being considered overlapping, but changing the field name does not, so it appears that type-identity for NamedTuples is determined solely by field type and order.

This is python 3.6.4 and mypy 0.700. I have not attempted to reproduce using git-master.

repro:

# foobar.py
from typing import NamedTuple, overload


A = NamedTuple("A", (("f", int),))
B = NamedTuple("B", (("q", int),))


@overload
def plain(a: A) -> int:
    ...

@overload
def plain(a: B) -> str:
    ...

def plain(a):
    if isinstance(a, A):
        return 4
    elif isinstance(a, B):
        return "4"
    else:
        raise TypeError()

resulting mypy error:

foobar.py:9: error: Overloaded function signatures 1 and 2 overlap with incompatible return types

[neilvyas]

I discovered this issue while using @overload, hence its use.

[ilevkivskyi]

Yes, this looks like a bug, named tuples should be considered as nominal.

Probably the overlap checks are missing the tuple fallback somewhere. If this is the case, then this should be not hard to fix.

cc @Michael0x2a who previously helped a lot with overloads.