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This fixes #16512. There are lots of parts of this patch: * The main payload is in FamInst. See Note [Coverage condition for injective type families] there for the overview. But it doesn't fix the bug. * We now bump the reduction depth every time we discharge a CFunEqCan. See Note [Flatten when discharging CFunEqCan] in TcInteract. * Exploration of this revealed a new, easy to maintain invariant for CTyEqCans. See Note [Almost function-free] in TcRnTypes. * We also realized that type inference for injectivity was a bit incomplete. This means we exchanged lookupFlattenTyVar for rewriteTyVar. See Note [rewriteTyVar] in TcFlatten. The new function is monadic while the previous one was pure, necessitating some faff in TcInteract. Nothing too bad. * zonkCt did not maintain invariants on CTyEqCan. It's not worth the bother doing so, so we just transmute CTyEqCans to CNonCanonicals. * The pure unifier was finding the fixpoint of the returned substitution, even when doing one-way matching (in tcUnifyTysWithTFs). Fixed now. Test cases: typecheck/should_fail/T16512{a,b}
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