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Simon Peyton Jones authored
This is a pretty big patch, but which substantially iproves the subsumption check. Trac #9569 was the presenting example, showing how type inference could depend rather delicately on eta expansion. But there are other less exotic examples; see Note [Co/contra-variance of subsumption checking] in TcUnify. The driving change is to TcUnify.tcSubType. But also * HsWrapper gets a new constructor WpFun, which behaves very like CoFun: if wrap1 :: exp_arg <= act_arg wrap2 :: act_res <= exp_res then WpFun wrap1 wrap2 : (act_arg -> arg_res) <= (exp_arg -> exp_res) * I generalised TcExp.tcApp to call tcSubType on the result, rather than tcUnifyType. I think this just makes it consistent with everything else, notably tcWrapResult. As usual I ended up doing some follow-on refactoring * AmbigOrigin is gone (in favour of TypeEqOrigin) * Combined BindPatSigCtxt and PatSigCxt into one * Improved a bit of error message generation
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