130a794c2b
## Summary Add two simple tests that we recently discussed with @dcreager. They demonstrate that the `TypeMapping::MarkTypeVarsInferable` operation really does need to keep track of the binding context. ## Test Plan Made sure that those tests fail if we create `TypeMapping::MarkTypeVarsInferable(None)`s everywhere.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.