gistfile1.txt

[jkoppel@MacBook-Pro-5:gadt_failed/Foo]$ stack build --ghc-options="-Wall"                                                         (06-22 01:30)
Foo-0.1.0.0: unregistering (local file changes: ChangeLog.md Foo.cabal README.md app/Main.hs package.yaml src/Lib.hs)
Building all executables for `Foo' once. After a successful build of all of them, only specified executables will be rebuilt.
Foo-0.1.0.0: configure (lib + exe)
Configuring Foo-0.1.0.0...
Foo-0.1.0.0: build (lib + exe)
Preprocessing library for Foo-0.1.0.0..
Building library for Foo-0.1.0.0..
[1 of 2] Compiling Lib              ( src/Lib.hs, .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Lib.o )
[2 of 2] Compiling Paths_Foo        ( .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/autogen/Paths_Foo.hs, .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Paths_Foo.o )
Preprocessing executable 'Foo-exe' for Foo-0.1.0.0..
Building executable 'Foo-exe' for Foo-0.1.0.0..
[1 of 2] Compiling Main             ( app/Main.hs, .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Foo-exe/Foo-exe-tmp/Main.o )

/Users/jkoppel/tmp/gadt_failed/Foo/app/Main.hs:22:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘fun’: Patterns not matched: (Inl _)
   |
22 | fun (Inr Bar) = 1
   | ^^^^^^^^^^^^^^^^^
[2 of 2] Compiling Paths_Foo        ( .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Foo-exe/autogen/Paths_Foo.hs, .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Foo-exe/Foo-exe-tmp/Paths_Foo.o )
Linking .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Foo-exe/Foo-exe ...

Foo-0.1.0.0: copy/register
Installing library in /Users/jkoppel/tmp/gadt_failed/Foo/.stack-work/install/x86_64-osx/lts-11.14/8.2.2/lib/x86_64-osx-ghc-8.2.2/Foo-0.1.0.0-GuLELn5n2LlHCGF6kEczSO
Installing executable Foo-exe in /Users/jkoppel/tmp/gadt_failed/Foo/.stack-work/install/x86_64-osx/lts-11.14/8.2.2/bin
Registering library for Foo-0.1.0.0..

Main.hs

{-# LANGUAGE TypeOperators, GADTs, Strict #-}

module Main where

data (:+:) f g a = Inl !(f a) | Inr !(g a)

data A
data B

data Foo l where
  Foo :: Foo A

data Bar l where
  Bar :: Bar B

type Sig = Foo :+: Bar

fun :: Sig B -> Int
fun (Inr Bar) = 1

main :: IO ()
main = return ()