Inconsistency w.r.t. coverage checking warnings for EmptyCase under unsatisfiable constraints
Consider the following code:
{-# 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 adb565aa), 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.
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |