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This PR carries over some of the `has_relation_to` logic for comparing a typevar with itself. A typevar will specialize to the same type if it's mentioned multiple times, so it is always assignable to and a subtype of itself. (Note that typevars can only specialize to fully static types.) This is also true when the typevar appears in a union on the right-hand side, or in an intersection on the left-hand side. Similarly, a typevar is always disjoint from its negation, so when a negated typevar appears on the left-hand side, the constraint set is never satisfiable. (Eventually this will allow us to remove the corresponding clauses from `has_relation_to`, but that can't happen until more of #20093 lands.)
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.