[commit: ghc] master: Fix #14991. (d8d4266)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/d8d4266bf73790f65b223ec16f645763eaed8be3/ghc

>---------------------------------------------------------------

commit d8d4266bf73790f65b223ec16f645763eaed8be3
Author: Richard Eisenberg <rae at cs.brynmawr.edu>
Date:   Mon Apr 2 15:32:04 2018 -0400

    Fix #14991.
    
    It turns out that solveEqualities really does need to use simpl_top.
    I thought that solveWanteds would be enough, and no existing test
    case showed up the different. #14991 shows that we need simpl_top.
    Easy enough to fix.
    
    test case: dependent/should_compile/T14991


>---------------------------------------------------------------

d8d4266bf73790f65b223ec16f645763eaed8be3
 compiler/typecheck/TcSimplify.hs                   |  4 ++-
 compiler/typecheck/TcTyClsDecls.hs                 |  2 +-
 testsuite/tests/dependent/should_compile/T14991.hs | 34 ++++++++++++++++++++++
 testsuite/tests/dependent/should_compile/all.T     |  1 +
 4 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/compiler/typecheck/TcSimplify.hs b/compiler/typecheck/TcSimplify.hs
index 7307f74..ccb7ef5 100644
--- a/compiler/typecheck/TcSimplify.hs
+++ b/compiler/typecheck/TcSimplify.hs
@@ -165,7 +165,9 @@ solveEqualities thing_inside
   = checkNoErrs $  -- See Note [Fail fast on kind errors]
     do { (result, wanted) <- captureConstraints thing_inside
        ; traceTc "solveEqualities {" $ text "wanted = " <+> ppr wanted
-       ; final_wc <- runTcSEqualities $ solveWanteds wanted
+       ; final_wc <- runTcSEqualities $ simpl_top wanted
+          -- NB: Use simpl_top here so that we potentially default RuntimeRep
+          -- vars to LiftedRep. This is needed to avoid #14991.
        ; traceTc "End solveEqualities }" empty
 
        ; traceTc "reportAllUnsolved {" empty
diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index 6598942..cdcc3bd 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -1485,7 +1485,7 @@ So, the kind-checker must return both the new args (that is, Type
 
 Because we don't need this information in the kind-checking phase of
 checking closed type families, we don't require these extra pieces of
-information in tc_fam_ty_pats. See also Note [tc_fam_ty_pats vs tcFamTyPats].
+information in tc_fam_ty_pats.
 
 Note [Failing early in kcDataDefn]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/testsuite/tests/dependent/should_compile/T14991.hs b/testsuite/tests/dependent/should_compile/T14991.hs
new file mode 100644
index 0000000..f435c37
--- /dev/null
+++ b/testsuite/tests/dependent/should_compile/T14991.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UndecidableInstances #-}
+module T14991 where
+
+import Data.Kind
+
+type family Promote (k :: Type) :: Type
+type family PromoteX (a :: k) :: Promote k
+
+type family Demote (k :: Type) :: Type
+type family DemoteX (a :: k) :: Demote k
+
+-----
+-- Type
+-----
+
+type instance Demote Type = Type
+type instance Promote Type = Type
+
+type instance DemoteX (a :: Type) = Demote a
+type instance PromoteX (a :: Type) = Promote a
+
+-----
+-- Arrows
+-----
+
+data TyFun :: Type -> Type -> Type
+type a ~> b = TyFun a b -> Type
+infixr 0 ~>
+
+type instance Demote  (a ~> b) = DemoteX  a -> DemoteX  b
+type instance Promote (a -> b) = PromoteX a ~> PromoteX b
diff --git a/testsuite/tests/dependent/should_compile/all.T b/testsuite/tests/dependent/should_compile/all.T
index 070e120..701e187 100644
--- a/testsuite/tests/dependent/should_compile/all.T
+++ b/testsuite/tests/dependent/should_compile/all.T
@@ -34,3 +34,4 @@ test('T14556', normal, compile, [''])
 test('T14720', normal, compile, [''])
 test('T14066a', normal, compile, [''])
 test('T14749', normal, compile, [''])
+test('T14991', normal, compile, [''])