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Simon Peyton Jones authored
When we spot an equality arising from a functional dependency, we now use that equality (a "wanted") to rewrite the work-item constraint right away. This avoids two dangers Danger 1: If we send the original constraint on down the pipeline it may react with an instance declaration, and in delicate situations (when a Given overlaps with an instance) that may produce new insoluble goals: see Trac #4952 Danger 2: If we don't rewrite the constraint, it may re-react with the same thing later, and produce the same equality again --> termination worries. To achieve this required some refactoring of FunDeps.lhs (nicer now!). This patch also contains a couple of unrelated improvements * A bad bug in TcSMonad.nestImplicTcS whereby the Tcs tyvars of an outer implication were not untouchable inside * Improved logging machinery for the type constraint solver; use -ddump-cs-trace (probably with a wider default line width -dppr-cols=200 or something)
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