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Summary: It's rather unfortunate that derived code can produce inaccessible code warnings (as demonstrated in #8128, #8740, and #15398), since the programmer has no control over the generated code. This patch aims to suppress `-Winaccessible-code` in all derived code. It accomplishes this by doing the following: * Generalize the `ic_env :: TcLclEnv` field of `Implication` to be of type `Env TcGblEnc TcLclEnv` instead. This way, it also captures `DynFlags`, which record the flag state at the time the `Implication` was created. * When typechecking derived code, turn off `-Winaccessible-code`. This way, any insoluble given `Implication`s that are created when typechecking this derived code will remember that `-Winaccessible-code` was disabled. * During error reporting, consult the `DynFlags` of an `Implication` before making the decision to report an inaccessible code warning. Test Plan: make test TEST="T8128 T8740 T15398" Reviewers: simonpj, bgamari Reviewed By: simonpj Subscribers: monoidal, rwbarton, thomie, carter GHC Trac Issues: #8128, #8740, #15398 Differential Revision: https://phabricator.haskell.org/D4993
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