[soaring0616]

Summary

For a concrete root type (not Any or a type variable), Pyrefly widens the root parameter of the synthesized RootModel.__init__ to Any when strict mode is disabled (pydantic.rs#L165-L169). This allows valid Pydantic runtime coercions (e.g., passing bytes to RootModel[str]) without triggering false positives, preserving root to the concrete type only when strict mode is enabled.

Previously, has_strict was computed in get_pydantic_root_model_type_via_mro (L131-L133), which only captures strictness written on the base class type argument (such as RootModel[StrictInt]), including when inherited from a parent class. As a result, model-level strict=True from model_config or a class keyword was ignored even though it is already resolved into dataclass.kws.strict.

This change takes dataclass.kws.strict into account when synthesizing RootModel.__init__, reusing the dataclass config already available there. This change also adds a test case covering the model_config, class keyword, inherited, and lax cases.

Fixes #5242

• I am a human, and I fully understand the code I am submitting.

What changed

• pyrefly/lib/alt/class/dataclass.rs:L156:160: Passed dataclass.kws.strict alongside has_strict when calling get_pydantic_root_model_init.
• pyrefly/lib/test/pydantic/root_model.rs:L353:373: Added test_root_model_config_strict with test classes covering model_config (class Text), class keyword (class Text2), inherited strictness (class Child), and the default lax behavior (class Lax).

Test Plan

• cargo test -p pyrefly --lib -- test::pydanticshows test result: ok. 174 passed; 0 failed; 0 ignored in my local machine
  -test_root_model_config_strict to fail with expected 3 errors, but got 0.
• Full test suite (test.py): On macOS debug builds, test::cycles::deep_phi_chain_term_expr hits a pre-existing stack overflow (reproduced without this change); the suite passes with PYREFLY_STACK_SIZE=33554432.
• Formatting & lints: cargo fmt --check and cargo clippy pass with no warnings.

AI disclosure

• I used Claude to study the codebase, provides the possible direction and the test range.
• I repeatedly test my ideas and confirm the direction, and verify them on my local machine.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅