Error on dict construction from iterable items

Bug Report

Attempting to construct a mapping from objects that are designed to be constructed into dict items results in an error.

To Reproduce

Run mypy on this file:

from typing import Dict


class MyObject(str):
    """
    >>> tuple(MyObject('G'))
    ('G', 71)
    """
    def __iter__(self):
        return iter((self, ord(self)))


def get_mapping() -> Dict[str, int]:
    """
    >>> get_mapping()
    {'a': 97, 'b': 98}
    """
    items = MyObject('a'), MyObject('b')
    return dict(items)

In this file, MyObject implements __iter__ to return a key/value pair such that a dict can be constructed from an iterable of MyObjects.

Note that python -m doctest dictable.py passes.

Expected Behavior

Mypy should honor dict's behavior to produce pairs from objects if they're iterable and accept any iterable of objects that when iterated on produce pairs of compatible types.

Actual Behavior

dictable.py:18: error: Argument 1 to "dict" has incompatible type "Tuple[MyObject, MyObject]"; expected "Iterable[Tuple[str, int]]"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.0
• Operating system and version: macOS 11.0

Start by running mypy on the reproduced example in dictable.py and compare it with the passing doctest behavior. Trace how the dict construction is typed, then verify that compatible iterable item objects are accepted while the existing type-checking behavior remains intact.