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Ryan Scott authored
Summary: Both #9692 and #14179 were caused by GHC being careless about using eta-reduced data family instance axioms. Each of those tickets were fixed by manually whipping up some code to eta-expand the axioms. The same sort of issue has now caused #15845, so I figured it was high time to factor out the code that each of these fixes have in common. This patch introduces the `etaExpandFamInstLHS` function, which takes a family instance's type variables, LHS types, and RHS type, and returns type variables and LHS types that have been eta-expanded if necessary, in the case of a data family instance. (If it's a type family instance, `etaExpandFamInstLHS` just returns the supplied type variables and LHS types unchanged). Along the way, I noticed that many references to `Note [Eta reduction for data families]` (in `FamInstEnv`) had slightly bitrotted (they either referred to a somewhat different name, or claimed that the Note lived in a different module), so I took the liberty of cleaning those up. Test Plan: make test TEST="T9692 T15845" Reviewers: goldfire, bgamari Reviewed By: goldfire Subscribers: rwbarton, carter GHC Trac Issues: #15845 Differential Revision: https://phabricator.haskell.org/D5294
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