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ec3d5ebd - [ty] Upcast heterogeneous and mixed tuples to homogeneous tuples where it's necessary to solve a `TypeVar` (#19635)

[ty] Upcast heterogeneous and mixed tuples to homogeneous tuples where it's necessary to solve a `TypeVar` (#19635) ## Summary This PR improves our generics solver such that we are able to solve the `TypeVar` in this snippet to `int | str` (the union of the elements in the heterogeneous tuple) by upcasting the heterogeneous tuple to its pure-homogeneous-tuple supertype: ```py def f[T](x: tuple[T, ...]) -> T: return x[0] def g(x: tuple[int, str]): reveal_type(f(x)) ``` ## Test Plan Mdtests. Some TODOs remain in the mdtest regarding solving `TypeVar`s for mixed tuples, but I think this PR on its own is a significant step forward for our generics solver when it comes to tuple types. --------- Co-authored-by: Douglas Creager <dcreager@dcreager.net>