This adds a new `Type` variant for holding an instance of a typevar
inside of a generic function or class. We don't handle specializing the
typevars yet, but this should implement most of the typing rules for
inside the generic function/class, where we don't know yet which
specific type the typevar will be specialized to.
This PR does _not_ yet handle the constraint that multiple occurrences
of the typevar must be specialized to the _same_ time. (There is an
existing test case for this in `generics/functions.md` which is still
marked as TODO.)
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>
## Summary
This PR closes#16248.
If the return type of the function isn't assignable to the one
specified, an `invalid-return-type` error occurs.
I thought it would be better to report this as a different kind of error
than the `invalid-assignment` error, so I defined this as a new error.
## Test Plan
All type inconsistencies in the test cases have been replaced with
appropriate ones.
---------
Co-authored-by: Carl Meyer <carl@astral.sh>
To kick off the work of supporting generics, this adds many new
(currently failing) tests, showing the behavior we plan to support.
This is still missing a lot! Not included:
- typevar tuples
- param specs
- variance
- `Self`
But it's a good start! We can add more failing tests for those once we
tackle these.
---------
Co-authored-by: Carl Meyer <carl@astral.sh>
