Support typechecking of type literals in backpack

Backpack is unable to type check signatures that expect a data which
is a type level literal. This was reported in issue #15138. These
commits are a fix for this. It also includes a minimal test case that
was mentioned in the issue.

Reviewers: bgamari, ezyang, goldfire

Reviewed By: bgamari, ezyang

Subscribers: simonpj, ezyang, rwbarton, thomie, carter

GHC Trac Issues: #15138

Differential Revision: https://phabricator.haskell.org/D4951

3 files changed, 38 insertions, 0 deletions

diff --git a/testsuite/tests/backpack/should_run/T15138.bkp b/testsuite/tests/backpack/should_run/T15138.bkp
new file mode 100644
index 0000000000..7cb9eeb804
--- /dev/null
+++ b/testsuite/tests/backpack/should_run/T15138.bkp
@@ -0,0 +1,36 @@
+{-# LANGUAGE ConstraintKinds             #-}
+{-# LANGUAGE DataKinds                   #-}
+{-# LANGUAGE KindSignatures              #-}
+{-# LANGUAGE MultiParamTypeClasses       #-}
+{-# LANGUAGE FlexibleInstances           #-}
+{-# LANGUAGE TypeFamilies                #-}
+{-# LANGUAGE FlexibleContexts            #-}
+
+unit indef where
+
+   signature Abstract where
+     import GHC.TypeLits
+     data NatType :: Nat
+
+   module Util where
+     import Abstract
+     import Data.Proxy
+     import GHC.TypeLits
+
+     natTypeToInteger :: KnownNat NatType => Proxy NatType -> Integer
+     natTypeToInteger = natVal
+
+unit concrete where
+   module Concrete where
+     type NatType = 32
+
+
+unit main where
+   dependency indef[Abstract=concrete:Concrete] (Util as MyUtil)
+
+   module Main where
+     import Data.Proxy
+     import MyUtil
+
+     main :: IO ()
+     main = do print $ natTypeToInteger Proxy
diff --git a/testsuite/tests/backpack/should_run/T15138.stdout b/testsuite/tests/backpack/should_run/T15138.stdout
new file mode 100644
index 0000000000..f5c89552bd
--- /dev/null
+++ b/testsuite/tests/backpack/should_run/T15138.stdout
@@ -0,0 +1 @@
+32
diff --git a/testsuite/tests/backpack/should_run/all.T b/testsuite/tests/backpack/should_run/all.T
index 48ed0c6310..61277b8f31 100644
--- a/testsuite/tests/backpack/should_run/all.T
+++ b/testsuite/tests/backpack/should_run/all.T
@@ -8,3 +8,4 @@ test('bkprun07', normal, backpack_run, [''])
 test('bkprun08', normal, backpack_run, [''])
 test('bkprun09', normal, backpack_run, ['-O'])
 test('T13955', normal, backpack_run, [''])
+test('T15138', normal, backpack_run, [''])