Fix #14869 by being more mindful of Type vs. Constraint

Summary:
Before, we were using `isLiftedTypeKind` in `reifyType`
before checking if a type was `Constraint`. But as it turns out,
`isLiftedTypeKind` treats `Constraint` the same as `Type`, so every
occurrence of `Constraint` would be reified as `Type`! To make things
worse, the documentation for `isLiftedTypeKind` stated that it
treats `Constraint` //differently// from `Type`, which simply isn't
true.

This revises the documentation for `isLiftedTypeKind` to reflect
reality, and defers the `isLiftedTypeKind` check in `reifyType` so
that it does not accidentally swallow `Constraint`.

Test Plan: make test TEST=T14869

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14869

Differential Revision: https://phabricator.haskell.org/D4474

1 files changed, 25 insertions, 0 deletions

diff --git a/testsuite/tests/th/T14869.hs b/testsuite/tests/th/T14869.hs
new file mode 100644
index 0000000000..c58d4e2720
--- /dev/null
+++ b/testsuite/tests/th/T14869.hs
@@ -0,0 +1,25 @@
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeInType #-}
+module T14869 where
+
+import Data.Kind
+import GHC.Exts
+import Language.Haskell.TH (pprint, reify, stringE)
+
+type MyConstraint = Constraint
+type MyLiftedRep  = LiftedRep
+
+type family Foo1 :: Type
+type family Foo2 :: Constraint
+type family Foo3 :: MyConstraint
+type family Foo4 :: TYPE MyLiftedRep
+
+$(pure [])
+
+foo1, foo2, foo3 :: String
+foo1 = $(reify ''Foo1 >>= stringE . pprint)
+foo2 = $(reify ''Foo2 >>= stringE . pprint)
+foo3 = $(reify ''Foo3 >>= stringE . pprint)
+foo4 = $(reify ''Foo4 >>= stringE . pprint)