wrong warning on incomplete case analysis in conjunction with empty data declarations

I load the following code into GHCi with warnings enabled (-Wall)

{-# LANGUAGE EmptyDataDecls #-}

data Empty

data EitherStrict a b = LeftStrict !a | RightStrict !b

test :: Char
test =
   case undefined :: EitherStrict Empty Char of
--      LeftStrict _ -> undefined
      RightStrict b -> b

I get the warning:

CaseOfEmptyData.hs:9:3:
    Warning: Pattern match(es) are non-exhaustive
             In a case alternative: Patterns not matched: LeftStrict _

I think this warning is wrong, since the LeftStrict case cannot happen. Am I right?

This may be related to #2431 (closed).

This problem is of minor priority for me. I found that in the original problem EmptyDataDecls would not helped me at all, and I found a better solution in this particular case.

Trac metadata

Trac field	Value
Version	6.12.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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changed weight to 5

added Tbug Trac import labels

changed weight to 0

Well this is very delicate. Yes, since

• The data type is empty
• The LeftStrict constructor is strict

I agree that a pattern (LeftStrict a) can never match

But does this come up often enough to be worth special-casing in the compiler.

I'll mark it priority lowest, but if it becomes importand for someone, do yell.

Trac metadata

Trac field	Value
Priority	normal → lowest

Trac metadata

Trac field	Value
TypeOfFailure	OtherFailure → IncorrectWarning
Component	Compiler → Compiler (Type checker)

added Plow label

The new pattern match checker handles empty strict fields correctly.

T15305 is a regression test.

closed