Re: [GHC] #14098: Incorrect pattern match warning on nested GADTs

#14098: Incorrect pattern match warning on nested GADTs -------------------------------------+------------------------------------- Reporter: jmgrosen | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #11984 | Differential Rev(s): Phab:D4434 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"a2d03c69b782212e6c476cfc1870bae493a4ac89/ghc" a2d03c6/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="a2d03c69b782212e6c476cfc1870bae493a4ac89" Fix the coverage checker's treatment of existential tyvars Previously, the pattern-match coverage checker was far too eager to freshen the names of existentially quantified type variables, which led to incorrect sets of type constraints that misled GHC into thinking that certain programs that involve nested GADT pattern matches were non-exhaustive (when in fact they were). Now, we generate extra equality constraints in the ConCon case of the coverage algorithm to ensure that these fresh tyvars align with existing existential tyvars. See `Note [Coverage checking and existential tyvars]` for the full story. Test Plan: make test TEST="T11984 T14098" Reviewers: gkaracha, bgamari, simonpj Reviewed By: simonpj Subscribers: simonpj, rwbarton, thomie, carter GHC Trac Issues: #11984, #14098 Differential Revision: https://phabricator.haskell.org/D4434 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14098#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler