Make DeriveFunctor work with unboxed tuples

Summary:
Unboxed tuples have `RuntimeRep` arguments which `-XDeriveFunctor` was
mistaking for actual data constructor arguments. As a result, a derived
`Functor` instance for a datatype that contained an unboxed tuple would
generate twice as many arguments as it needed for an unboxed tuple pattern
match or expression. The solution is to simply put `dropRuntimeRepArgs` in the
right place.

Fixes #12399.

Test Plan: ./validate

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie, osa1

Differential Revision: https://phabricator.haskell.org/D2404

GHC Trac Issues: #12399

compiler/typecheck/TcGenDeriv.hs

@@ -1701,7 +1701,11 @@ functorLikeTraverse var (FT { ft_triv = caseTrivial,     ft_var = caseVar
                           = (caseTyApp fun_ty (last xrs), True)
        | otherwise        = (caseWrongArg, True)   -- Non-decomposable (eg type function)
        where
-         (xrs,xcs) = unzip (map (go co) args)
+         -- When folding over an unboxed tuple, we must explicitly drop the
+         -- runtime rep arguments, or else GHC will generate twice as many
+         -- variables in a unboxed tuple pattern match and expression as it
+         -- actually needs. See Trac #12399
+         (xrs,xcs) = unzip (map (go co) (dropRuntimeRepArgs args))
     go co (ForAllTy (TvBndr v vis) x)
        | isVisibleArgFlag vis = panic "unexpected visible binder"
        | v /= var && xc       = (caseForAll v xr,True)
@@ -2813,7 +2817,7 @@ a is the last type variable in a given datatype):
 * ft_tup:     A tuple type which mentions the last type variable in at least
               one of its fields. The TyCon argument of ft_tup represents the
               particular tuple's type constructor.
-              Examples: (a, Int), (Maybe a, [a], Either a Int)
+              Examples: (a, Int), (Maybe a, [a], Either a Int), (# Int, a #)
 
 * ft_ty_app:  A type is being applied to the last type parameter, where the
               applied type does not mention the last type parameter (if it

testsuite/tests/deriving/should_compile/T12399.hs

+{-# LANGUAGE DeriveFunctor, MagicHash, UnboxedTuples #-}
+module T12399 where
+
+import GHC.Exts
+
+newtype RmLoopsM a = RmLoopsM { runRmLoops :: Int# -> (# Int#, a #) }
+  deriving Functor

testsuite/tests/deriving/should_compile/all.T

@@ -71,3 +71,4 @@ test('T11732b', normal, compile, [''])
 test('T11732c', normal, compile, [''])
 test('T11833', normal, compile, [''])
 test('T12245', normal, compile, [''])
+test('T12399', normal, compile, [''])