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Incorrect type narrowing via isinstance after temporary type narrowing on initial object

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Dal 6/10/2026.

  • #5112 di @tonyyuyiding — aperta

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
python, rust
Ambito
compilers, devtools

Direzione di ricerca

Start by reproducing the reported narrowing behavior in the linked Pyrefly sandbox, focusing on the foo and foo2 examples and their reveal_type results. Trace the type-checker's handling of isinstance and attribute narrowing; done means the final narrowing results and bar diagnostics match the expectations shown in the issue.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

narrowing typechecking
Describe the Bug

I have a function in which I pass some object with an attribute x of class A. A.x has type int | None. A has a subclass B which narrows the type of B.x to just int.

I can use an isinstance(o, B) check to narrow o.x to int.

However after checking if o.x where o is not narrowed to B, the instance check fails to apply any further narrowing, regardless of what was done with the if o.x check.

Checking if o.x while o is narrowed to B has no such effect.

This snippet illustrates the problem:

from typing import reveal_type
import dataclasses


@dataclasses.dataclass(frozen=True)
class A:
    x: int | None
    y: int


@dataclasses.dataclass(frozen=True)
class B(A):
    x: int
    y: int | None


def foo(o: A):
    reveal_type(o.x)  # int | None <- correct
    bar(o.x)  # error <- correct
    if isinstance(o, B):
        reveal_type(o.x)  # int <- correct
        bar(o.x)  # no error <- correct

        if o.x: pass

        reveal_type(o.x)  # int <- correct
        bar(o.x)  # no error <- correct
    
    if o.x: pass

    reveal_type(o.x)  # int | None <- correct
    bar(o.x)  # error <- correct

    if isinstance(o, B):
        reveal_type(o.x)  # int | None <- incorrect. correct: int
        bar(o.x)  # error <- incorrect
    
    if o.x: return

    reveal_type(o.x)  # Literal[0] | None <- correct.
    bar(o.x)  # error <- correct

    if isinstance(o, B):
        reveal_type(o.x)  # Literal[0] | None <- incorrect. correct: int
        bar(o.x)  # error <- incorrect


def bar(x: int):
    pass

The same thing happens for less narrow types in a subclass (which yields a bad-override error on definition, but still shouldn't behave like this).

def foo2(o: A):
    reveal_type(o.y)  # int <- correct
    bar(o.y)  # no error <- correct
    if isinstance(o, B):
        reveal_type(o.y)  # int | None <- correct
        bar(o.y)  # error <- correct

        if o.y: pass
        
        reveal_type(o.y)  # int | None <- correct
        bar(o.y)  # error <- correct
    
    if o.y: pass

    if isinstance(o, B):
        reveal_type(o.y)  # int <- incorrect. correct: int | None
        bar(o.y)  # no error <- incorrect
    
    if o.y: return

    reveal_type(o.y)  # Literal[0] <- correct.
    bar(o.y)  # no error <- correct

    if isinstance(o, B):
        reveal_type(o.y)  # Literal[0] <- incorrect. correct: Literal[0] | None
        bar(o.y)  # no error <- incorrect

I first encountered this bug with pyrefly 1.2.0, but I confirmed that is still occurs with 1.3.1.

Sandbox Link

https://pyrefly.org/sandbox/?project=v2.tVZNT8MwDP0rlnpokaBBcKvgsP0C7ohDBdXYpUHtQBu_HidO0nyzruttyhy_2H7PfTNWprNn_C3QSYXaGu3G2vyuMN1v1z-LceBsSIcb1cxjg4PU_KajkzyanXFbbTSBKamdzQAYnXFe8QbMFau-itfHG4DCugdPd7jIBjExxad2MGFyukGEKzJ-C1tXXnE4P4uP1fMI3BSKoBjZKHavBCeCTI0B3FUbuaSVE-C-VwlrndniR_IR5lasYlqPuZqVTXi9fwtLr9esPQq8uAVaNyKO5KXfoYdvKeshI61Tknz0BPo_Sbw5_ZigEuTzUf-VFi4TKlefrgjtkc5AL-lDmgnOBs5NIy0LfGFOFpTKYmdCDxkCXFq6CxrrQCCaWY0I3UABL3QA6sOKUGgjdt_ottEeYMEFGu0OPg-Hr7FhTF_nw451Pfvg7yNzbjDU1QDtT7uXHt5NB1y6lxHTxm3I41k2pCQXUuZMSCk8CEZkfcUf

(Only applicable for extension issues) IDE Information

No response

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Rust
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