Pydantic lax-mode rewrites `Pattern[str]` to `Pattern[LaxStr]` and rejects `re.Pattern[str]`

[oriori1703]

Describe the Bug

Summary

When checking Pydantic models in Pyrefly, constructor parameters for re.Pattern[str] fields appear to be rewritten as Pattern[LaxStr] in lax mode.
This then causes valid re.Pattern[str] values (from re.compile(...)) to be rejected by static checking.

This seems inconsistent with runtime Pydantic behavior and likely indicates a bug/edge case in lax-type conversion for re.Pattern[...].

Minimal repro

import re
from typing import Literal
import pydantic

class RegexSignature(pydantic.BaseModel):
    type: Literal["regex"] = "regex"
    signature: re.Pattern[str]

_ = RegexSignature(type="regex", signature=re.compile(r"needle"))

Pyrefly output

ERROR Argument `Pattern[str]` is not assignable to parameter `signature` with type `Pattern[LaxStr]` in function `RegexSignature.__init__` [bad-argument-type]

(From my project this occurs in many callsites with the same shape.)

Expected behavior

No type error for re.compile(...) when field type is re.Pattern[str].

Actual behavior

Pyrefly reports Pattern[str] is not assignable to Pattern[LaxStr].

Why this looks wrong

• Runtime Pydantic accepts both:
  • RegexSignature(signature=re.compile("x"))
  • RegexSignature(signature="x") (string pattern coercion)
• Runtime Pydantic does not accept bytes for this field (e.g. signature=b"x" fails validation), so expanding to LaxStr may be over-broad for Pattern[str].
• pyright and mypy report no errors on the same callsites.

Environment

• pyrefly 0.59.0
• pydantic 2.12.5
• Python 3.14.3
• Linux

Additional context

This appears related to Pyrefly’s Pydantic lax-mode conversion behavior:
https://pyrefly.org/en/docs/pydantic/#how-lax-mode-works-in-pyrefly

Potentially, re.Pattern[T] may need special handling instead of recursively applying str -> LaxStr inside the type parameter.

Important

Although I encountered and debugged the issue myself, I used AI to create the content of the issue. I then verified the content of the issue and checked that the reproduction is correct.

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[rchen152]

cc @migeed-z

[asukaminato0721]

maybe related #2505

[NathanTempest]

As of v60.0 the bug is still live and thanks for the well-documented report @oriori1703

re.Pattern needs special-casing in the lax-mode expansion to avoid invariance issues. The correct lax type for Pattern[str] is Pattern[str] | str, not Pattern[LaxStr].

PR #3001 from @asukaminato0721 looks like the right fix. cc @migeed-z for review.

[oriori1703]

Thanks ❤️