## Summary Part of #15382 This PR adds support for checking the assignability of two general callable types. This is built on top of #16804 by including the gradual parameters check and accepting a function that performs the check between the two types. ## Test Plan Update `is_assignable_to.md` with callable types section.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/red_knot_test/README.md for documentation of this test format.