Compiler warning about non-exhaustive pattern match with GADT and type-signature

[Ashley Yakeley]

This program:

{-# OPTIONS -Wall #-}
{-# LANGUAGE GADTs #-}
module Bug where

    data T n where
        BoolT :: T Bool
        IntT :: T Int
 
    f :: T Bool -> Int
    f BoolT = 3

...gives this warning

$ ghc -c Bug.hs 

Bug.hs:10:4:
    Warning: Pattern match(es) are non-exhaustive
             In the definition of `f': Patterns not matched: IntT

The pattern match is, in fact, exhaustive, given the type signature.

[simonpj]

Happily this already works in the HEAD (and hence in the upcoming 6.12 release)

Simon