Overridden `__iter__` with more specific item type is only respected in some contexts

[andersk]

Bug Report

Given a subclass C of Iterable[int] that more specifically implements def __iter__(self) -> Iterator[bool] (which is fine, since bool is a subclass of int), I would expect mypy to understand that iterating C() yields bools. And indeed it does understand this in some contexts, such as iter(C()) and for item in C(). But in other contexts, such as list(C()) and unpacking, it relies only on the base class Iterable[int] and discards the more specific information from __iter__.

To Reproduce

from typing import Iterable, Iterator

class C(Iterable[int]):
    def __iter__(self) -> Iterator[bool]:
        yield True

# these give bool, as expected
reveal_type(C().__iter__())  # Iterator[bool]
reveal_type(iter(C()))  # Iterator[bool]
reveal_type([comp_item for comp_item in C()])  # list[bool]
for loop_item in C(): reveal_type(loop_item)  # bool

# these unexpectedly give int
reveal_type(list(C()))  # list[int]
[unpack_item] = C(); reveal_type(unpack_item)  # int
[*unpack_items] = C(); reveal_type(unpack_items)  # list[int]

# these unexpectedly give Any
reveal_type([*C()])  # list[Any] (#15747)
reveal_type(max(*C()))  # Any

Playground link

Expected Behavior

All items should be inferred as bool.

main.py:8: note: Revealed type is "typing.Iterator[builtins.bool]"
main.py:9: note: Revealed type is "typing.Iterator[builtins.bool]"
main.py:10: note: Revealed type is "builtins.list[builtins.bool]"
main.py:11: note: Revealed type is "builtins.bool"
main.py:14: note: Revealed type is "builtins.list[builtins.bool]"
main.py:15: note: Revealed type is "builtins.bool"
main.py:16: note: Revealed type is "builtins.list[builtins.bool]"
main.py:19: note: Revealed type is "builtins.list[builtins.bool]"
main.py:20: note: Revealed type is "builtins.bool"

Actual Behavior

The items are inconsistently inferred as bool, int, or Any, depending on the context.

main.py:8: note: Revealed type is "typing.Iterator[builtins.bool]"
main.py:9: note: Revealed type is "typing.Iterator[builtins.bool]"
main.py:10: note: Revealed type is "builtins.list[builtins.bool]"
main.py:11: note: Revealed type is "builtins.bool"
main.py:14: note: Revealed type is "builtins.list[builtins.int]"
main.py:15: note: Revealed type is "builtins.int"
main.py:16: note: Revealed type is "builtins.list[builtins.int]"
main.py:19: note: Revealed type is "builtins.list[Any]"
main.py:20: note: Revealed type is "Any"

One of the Any cases was previously reported:

• Custom __iter__ method yields false positive when used with star args (*) #14470
• Unpacking an iterable in a list comprehension leads to the type inferred as list[Any] #15747

Your Environment

• Mypy version used: 1.7.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11

[flaeppe]

I think #14470 reports the same thing as well

[andersk]

Yeah #14470 and #15747 seem to be the same as each other. That’s just one part of this report though.