Foo and Union[Foo, bool] fail to unify, when working with NamedTuple and recursively typed values

[mikeyhew]

Obviously, this example is infinitely recursive. The original code that produced the error was more complicated so I distilled it down to the simplest example that still triggers the error.

from typing import Union, NamedTuple

class Foo(NamedTuple):
    bar: Union[Foo, bool]

def make_foo() -> Union[Foo, bool]:
    return Foo(make_foo())

# foo.py:7: error: Incompatible return value type (got "Foo", expected "Union[Foo, bool]")

Replacing the body of make_foo with the following does not get rid of the error, but produces an extra error which may or may not be unrelated.

bar: Union[Foo, bool] = make_foo()
return Foo(bar)

# foo.py:7: error: Incompatible types in assignment (expression has type "Union[Foo, bool]", variable has type "Union[Foo, bool]")
# foo.py:8: error: Incompatible return value type (got "Foo", expected "Union[Foo, bool]")````

[gvanrossum]

Maybe it thinks that there are two different types named Foo?

You can debug this more by putting calls to reveal_type() in various places (search the mypy docs for how).

[pkch]

The changes required to fix this will likely cause merge conflicts with #3025; so it's probably better to fix it either after that PR is merged in, or inside it.

[mikeyhew]

Using reveal_type, the types still look like they should be compatible.

reveal_type(make_foo)
# Revealed type is 'def () -> Union[Tuple[Union[foo.Foo, builtins.bool], fallback=foo.Foo], builtins.bool]'

reveal_type(Foo(True))
# Revealed type is 'Tuple[Union[foo.Foo, builtins.bool], fallback=foo.Foo]'

[ilevkivskyi]

This is also fixed by #3952 both examples now work as expected, however a warning about forward references not fully supported is (expectedly) emitted.