`BaseModel.model_copy()` seems to lose intersection data

[oriori1703]

Describe the Bug

I’m seeing what looks like a false positive bad-return when calling pydantic.BaseModel.model_copy() inside an isinstance branch on a generic type variable.

Notably, I included a control example with a similar copy(self) -> Self method (Copyable) that does type-check successfully in the same pattern. That suggests this is specific to pyrefly’s handling/integration of pydantic BaseModel.model_copy() rather than a general issue with Self + narrowed TypeVar.

Minimal repro

from copy import copy
from typing import Self, reveal_type

from pydantic import BaseModel


# Working example
class Copyable:
    def copy(self) -> Self:
        return copy(self)


class CopyableChild:
    pass


def test[T: Copyable](value: T) -> T:
    if isinstance(value, CopyableChild):
        reveal_type(value)  # revealed type is `CopyableChild & T`
        value_copy = value.copy()
        reveal_type(value_copy)  # revealed type is `CopyableChild & T`
        return value_copy
    else:
        reveal_type(value)
        return value


# Error reproduction
class ParenModel(BaseModel):
    field: int | str


class ChildModel(BaseModel):
    field: int


def test2[T: ParenModel](value: T) -> T:
    if isinstance(value, ChildModel):
        reveal_type(value)  # revealed type is `ChildModel & T`
        value_copy = value.model_copy()
        reveal_type(value_copy)  # revealed type is `ChildModel` instead of `ChildModel & T`
        return value_copy
    else:
        reveal_type(value)
        return value

pyrefly output:

> pyrefly check test.py
 INFO revealed type: CopyableChild & T [reveal-type]
  --> test.py:19:20
   |
19 |         reveal_type(value)  # revealed type is `CopyableChild & T`
   |                    -------
   |
 INFO revealed type: T [reveal-type]
  --> test.py:21:20
   |
21 |         reveal_type(value_copy)  # revealed type is `CopyableChild & T`
   |                    ------------
   |
 INFO revealed type: T [reveal-type]
  --> test.py:24:20
   |
24 |         reveal_type(value)
   |                    -------
   |
 INFO revealed type: ChildModel & T [reveal-type]
  --> test.py:39:20
   |
39 |         reveal_type(value)  # revealed type is `ChildModel & T`
   |                    -------
   |
 INFO revealed type: ChildModel [reveal-type]
  --> test.py:41:20
   |
41 |         reveal_type(value_copy)  # revealed type is `ChildModel` instead of `ChildModel & T`
   |                    ------------
   |
ERROR Returned type `ChildModel` is not assignable to declared return type `T` [bad-return]
  --> test.py:42:16
   |
37 | def test2[T: ParenModel](value: T) -> T:
   |                                       - declared return type
38 |     if isinstance(value, ChildModel):
39 |         reveal_type(value)  # revealed type is `ChildModel & T`
40 |         value_copy = value.model_copy()
41 |         reveal_type(value_copy)  # revealed type is `ChildModel` instead of `ChildModel & T`
42 |         return value_copy
   |                ^^^^^^^^^^
   |
 INFO revealed type: T [reveal-type]
  --> test.py:44:20
   |
44 |         reveal_type(value)
   |                    -------
   |
 INFO 1 error

Environment

• pyrefly 0.60.2
• Python 3.14.3
• pydantic 2.12.5

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[rchen152]

Thanks for the report! This doesn't seem to be pydantic-specific; I can reproduce the bug without pydantic like this:

from copy import copy
from typing import Self, reveal_type

class BaseModel:
    def model_copy(self) -> Self: ...

# Error reproduction
class ParentModel(BaseModel):
    field: int | str


class ChildModel(BaseModel):
    field: int


def test2[T: ParentModel](value: T) -> T:
    if isinstance(value, ChildModel):
        reveal_type(value)  # revealed type is `ChildModel & T`
        value_copy = value.model_copy()
        reveal_type(value_copy)  # revealed type is `ChildModel` instead of `ChildModel & T`
        return value_copy  # E: Returned type `ChildModel` is not assignable to declared return type `T`
    else:
        reveal_type(value)
        return value

sandbox

[oriori1703]

My bad 🤦

In my "working" example I forgot to inherit from Copyable .
If I change Change class CopyableChild to class CopyableChild(Copyable) I get the same error.

I hope you didn't waste too much time on that :(

Do you want me to edit the issue?

[rchen152]

No worries! We have a working minimal repro now, so it's all good :)