[GHC] #14098: Incorrect pattern match warning on nested GADTs
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Mon Aug 7 22:54:54 UTC 2017
#14098: Incorrect pattern match warning on nested GADTs
-------------------------------------+-------------------------------------
Reporter: jmgrosen | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.2.1
Keywords: | Operating System: Unknown/Multiple
PatternMatchWarnings |
Architecture: | Type of failure: Incorrect
Unknown/Multiple | error/warning at compile-time
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
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{{{#!hs
{-# Language GADTs #-}
data App f a where
App :: f a -> App f (Maybe a)
data Ty a where
TBool :: Ty Bool
TInt :: Ty Int
data T f a where
C :: T Ty (Maybe Bool)
-- Warning
f :: T f a -> App f a -> ()
f C (App TBool) = ()
-- No warning
g :: T f a -> App f a -> ()
g C (App x) = case x of
TBool -> ()
}}}
When compiling, with `-Wincomplete-patterns`:
{{{
Foo.hs:15:1: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In an equation for âfâ: Patterns not matched: C (App TInt)
|
15 | f C (App TBool) = ()
| ^^^^^^^^^^^^^^^^^^^^
}}}
I'm sorry for such a complicated example, but I wasn't able to reduce it
any further than this.
The gist of the problem is that this code gives a pattern matching non-
exhaustiveness warning when matching a nested pattern, when pulling out a
value then matching on it produces no warning (correctly). It also seems
to have to do with using a type constructor (`Maybe`) within the
constructor definition of `App`, as changing it to
{{{#!hs
data App f a where
App :: f a -> App f a
...
data T f a where
C :: T Ty Bool
}}}
does not give a warning, even when `App` is modified further to force it
to be a proper GADT.
This might be a known limitation of the checker, but given that it works
fine when the nesting is removed, I would think it would be more of a bug.
Thanks to Iavor for helping me minimize the test case.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14098>
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