#15450: Inconsistency w.r.t. coverage checking warnings for EmptyCase under unsatisfiable constraints -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple PatternMatchWarnings | Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: #12957 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following code: {{{#!hs {-# LANGUAGE EmptyCase #-} {-# LANGUAGE GADTs #-} {-# OPTIONS_GHC -Wincomplete-patterns #-} module Bug where f :: (Int ~ Bool) => Bool -> a f x = case x of {} g :: (Int ~ Bool) => Bool -> a g x = case x of True -> undefined }}} {{{ $ /opt/ghc/8.4.3/bin/ghci Bug.hs GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:10:7: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: False | 10 | g x = case x of True -> undefined | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} Observe that we get a non-exhaustivity warning for the `case` expression in `g` (thanks to commit adb565aa74582969bbcc3b411d6d518b1c76c3cf), but not for the one in `f`. The difference is that `f` uses `EmptyCase`, whereas `g` does not, and the codepath for `EmptyCase` is unfortunately incongruous with the codepath for other coverage checking. I know how to fix this. Patch incoming. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15450 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler