TypeGuard does not narrow when statement compares for identity (`is True`)

[flaeppe]

Bug Report

In the example below f is a TypeGuard that mypy fails to narrow the type with when the statement compares with identity (is True)

To Reproduce

https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=688d7597c968ed90ecdd8d4bf2b3a8a3

from typing import TypeGuard

def f(value: str) -> TypeGuard[int]:
    return True

# This should narrow to "builtins.int"
by_identity = "123"
assert f(by_identity) is True
reveal_type(by_identity)  # Revealed type is "builtins.str"

Expected Behavior

Revealed type is "builtins.int"

Actual Behavior

Revealed type is "builtins.str"

Your Environment

• Mypy version used: 1.10.1 (and master)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[sobolevn]

The same happens for

by_identity = "123"
assert f(by_identity) == True
reveal_type(by_identity)  # Revealed type is "builtins.str"

[sobolevn]

This happens because we only check for type guards here:

mypy/mypy/checker.py

Lines 5837 to 5839 in ec00fb8

	elif isinstance(node.callee, RefExpr):
	if node.callee.type_guard is not None or node.callee.type_is is not None:
	# TODO: Follow *args, **kwargs

But, is True / == True is checked in this branch:

mypy/mypy/checker.py

Line 5889 in ec00fb8

elif isinstance(node, ComparisonExpr):

It never really calls the first branch for its operands.

I think it would make sense to move the type guard part into a helper function and call it from both branches.

@flaeppe I can assign this on you if you want :)

[flaeppe]

@flaeppe I can assign this on you if you want :)

Sure, go ahead.

And thank you for the insight, I'll try to have a look

[DanielGardin]

Hey! I was messing around with TypeGuard and, for a similar reason (I guess), an if statement also does not narrow the type if checked by a TypeGuard function. This is a working example:
https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=2e8650f66d19008f73c45a4df37bcff0

from typing import Any, Iterable, TypeGuard, TypeVar

_T = TypeVar('_T')
def is_iterable_type(obj: Any, dtype: type[_T]) -> TypeGuard[Iterable[_T]]:
    return isinstance(obj, Iterable) and all(isinstance(item, dtype) for item in obj)

def if_narrow(iterable: str | Iterable[str]) -> None:
    if is_iterable_type(iterable, str):
        return None
    
    reveal_type(iterable)


def assert_narrow(iterable: str | Iterable[str]) -> None:
    assert is_iterable_type(iterable, str)
        
    reveal_type(iterable)


def assert_cmp_narrow(iterable: str | Iterable[str]) -> None:
    assert is_iterable_type(iterable, str) == True
        
    reveal_type(iterable)

The only one that correctly narrows down to str is the second one. I imagine this is already known, but this was the only issue I could find about a similar problem.

[JelleZijlstra]

@DanielGardin I would have expected if_narrow to narrow; if that doesn't, that seems like a bug that's separate from this issue.

I'd recommend using int instead of str to demonstrate this, since str is an Iterable[str], which complicates things. Here's your sample with int instead of str: https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=1e467dde5ab742afb0b00cb3135dc400

[erictraut]

TypeGuard does not narrow in the negative (else) case, so I think mypy's behavior is correct here. If you change your code to use TypeIs, it would work as you are expecting.