Fix TcUnify.tc_sub_type_ds

This patch fixes Trac #12616. There were two separate bugs

1.  For some reason, in TcDeriv we switched on
    -XImpredicativeTypes locally, for the code generated by a
    'deriving' clause.  But GHC really doesn't support
    impredicative types, so this is a deeply bogus thing to
    do.  And it interacted with (2) to cause the failure.

    So I've just removed the local flag setting.  Let's see if
    anything breaks.  (If it does, a non-solution is to
    restore the flag!)

2.  In TcUnify.tc_sub_type_ds we were going to some trouble to
    support co- and contra-variance even for impredicative
    types.  Because of (1) that allowed a unification variable
    to be unified with a polytype (probably wrongly) and that
    caused later trouble, in the constraint solver where
    -XImpredicativeTypes was not on.  In effect,
    -XImpredicativeTypes can't be switched on locally.

So this patch just deletes code to fix the bug.  I think we
should probably nuke -XImpredicativeTypes altogether.

2 files changed, 9 insertions, 24 deletions

diff --git a/compiler/typecheck/TcDeriv.hs b/compiler/typecheck/TcDeriv.hs
index 0b5f07301a..342e0c08a1 100644
--- a/compiler/typecheck/TcDeriv.hs
+++ b/compiler/typecheck/TcDeriv.hs
@@ -2272,8 +2272,7 @@ genInst spec@(DS { ds_tvs = tvs, ds_tc = rep_tycon
                         { ib_binds      = gen_Newtype_binds loc clas tvs tys rhs_ty
                         , ib_tyvars     = map Var.varName tvs   -- Scope over bindings
                         , ib_pragmas    = []
-                        , ib_extensions = [ LangExt.ImpredicativeTypes
-                                          , LangExt.RankNTypes ]
+                        , ib_extensions = [ LangExt.RankNTypes ]
                         , ib_derived    = True } }
                 , emptyBag
                 , Just $ getName $ head $ tyConDataCons rep_tycon ) }
diff --git a/compiler/typecheck/TcUnify.hs b/compiler/typecheck/TcUnify.hs
index b564f9fd9a..b8c7640223 100644
--- a/compiler/typecheck/TcUnify.hs
+++ b/compiler/typecheck/TcUnify.hs
@@ -721,28 +721,14 @@ tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected
                     ; tc_sub_type_ds eq_orig inst_orig ctxt ty_a' ty_e }
                Unfilled _   -> unify }
 
-
-    go ty_a (TyVarTy tv_e)
-      = do { dflags <- getDynFlags
-           ; tclvl  <- getTcLevel
-           ; lookup_res <- lookupTcTyVar tv_e
-           ; case lookup_res of
-               Filled ty_e' ->
-                 do { traceTc "tcSubTypeDS_NC_O following filled exp meta-tyvar:"
-                        (ppr tv_e <+> text "-->" <+> ppr ty_e')
-                    ; tc_sub_tc_type eq_orig inst_orig ctxt ty_a ty_e' }
-               Unfilled details
-                 |  canUnifyWithPolyType dflags details
-                    && isTouchableMetaTyVar tclvl tv_e  -- don't want skolems here
-                 -> unify
-
-     -- We've avoided instantiating ty_actual just in case ty_expected is
-     -- polymorphic. But we've now assiduously determined that it is *not*
-     -- polymorphic. So instantiate away. This is needed for e.g. test
-     -- typecheck/should_compile/T4284.
-                 |  otherwise
-                 -> inst_and_unify }
-
+    -- Historical note (Sept 16): there was a case here for
+    --    go ty_a (TyVarTy alpha)
+    -- which, in the impredicative case unified  alpha := ty_a
+    -- where th_a is a polytype.  Not only is this probably bogus (we
+    -- simply do not have decent story for imprdicative types), but it
+    -- caused Trac #12616 because (also bizarrely) 'deriving' code had
+    -- -XImpredicativeTypes on.  I deleted the entire case.
+    
     go (FunTy act_arg act_res) (FunTy exp_arg exp_res)
       | not (isPredTy act_arg)
       , not (isPredTy exp_arg)