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This aids with debugging, since all you have to do to get more stack frames is add a constraint `(?callStack :: CallStack) =>`. Old output: ``` ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20160107 for x86_64-unknown-linux): ASSERT failed! file compiler/types/TyCoRep.hs line 1800 InScope [] [Xuv :-> n_av5[sk]] [] ``` New output: ``` ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20160107 for x86_64-unknown-linux): ASSERT failed! CallStack (from ImplicitParams): assertPprPanic, called at compiler/types/TyCoRep.hs:1800:95 in ghc:TyCoRep InScope [] [Xuv :-> n_av5[sk]] [] ``` Test Plan: harbormaster manual testing Reviewers: austin, gridaphobe, bgamari Reviewed By: gridaphobe, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1751
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