Generic type is not inferred across descriptor class methods

[sjustas]

Tried to write type stubs for lazy package. Lazy provides a descriptor that invokes the underlying method only once and caches the result in the instance.

The inferred type is <nothing>, so mypy crashes when checking expressions.

from typing import Any, Generic, Callable, TypeVar

T = TypeVar('T')

class lazy:
    def __init__(self, func: Callable[[Any], T]) -> None: ...
    def __get__(self, inst: Any, inst_cls: Any) -> T: ...

class Haystack:
    @lazy
    def hasNeedles(self) -> bool:
        # some expensive lookup we want to execute only once
        return True

reveal_type(Haystack().hasNeedles)

# checking this expression crashes
x = Haystack().hasNeedles or True

<...>
note: Revealed type is '<nothing>'
<...>
version: 0.761
Traceback (most recent call last):
  File "mypy/checkexpr.py", line 3645, in accept
  File "mypy/nodes.py", line 1736, in accept
  File "mypy/checkexpr.py", line 2029, in visit_op_expr
  File "mypy/checkexpr.py", line 2662, in check_boolean_op

If lazy was defined as a function, type gets inferred correctly.

def lazy(func: Callable[[Any], T]) -> T: ...

note: Revealed type is 'builtins.bool*'

[sjustas]

Issues that might be related: #5738, #1317

[janverb]

This works as expected if you change lazy to inherit from Generic[T]. Crashing seems like the wrong response though.

[ethanhs]

We definitely should not crash. It might be nice if we have an unused type variable in a method to note the class may need to inherit from Generic.

[sjustas]

Thanks for such a quick response. You guys rock!
Yeah, a warning that class needs to be generic would've been useful.

[JukkaL]

This doesn't crash any more on master. Here's the output I get:

$ mypy t/t3.py
t/t3.py:7: error: A function returning TypeVar should receive at least one argument containing the same Typevar
t/t3.py:15: note: Revealed type is "<nothing>"
t/t3.py:18: error: Need type annotation for "x"
Found 2 errors in 1 file (checked 1 source file)