Re-add type equalities GHC claims spurious but are needed for typing

src/HordeAd/Core/Engine.hs

@@ -235,19 +235,20 @@ fwdArtifactAdapt f vals =
 
 -- * Old gradient adaptors, with constant and fixed inputs and dt
 
--- We are inlining these function, in part, because they take function
--- arguments and calling unknown functions is expensive and, in part,
--- because we can't easily specialize them due to
--- https://gitlab.haskell.org/ghc/ghc/-/issues/23798
--- However, they are called in many places, so we break the inline chain
--- at crevOnHVector to avoid binary blowup.
+-- The equality `RankedOf f ~ Flip OR.Array` is needed for type-checking
+-- later on, even though GHC 9.6.4 reports it as redundant.
+--
+-- We are inlining these functions because they take function arguments
+-- and are not too large. However, becausethey are called in many places,
+-- we break the inline chain at crevOnHVector, to avoid exe blowup.
 -- | The old versions that use the fixed input and @dt@ to compute gradient
 -- only at these values, both transposing and evaluating at the same time.
 --
 -- These work for @f@ both ranked and shaped.
 crev
   :: forall r y f vals advals.
-     ( AdaptableHVector (ADVal (Flip OR.Array)) advals
+     ( RankedOf f ~ Flip OR.Array
+     , AdaptableHVector (ADVal (Flip OR.Array)) advals
      , AdaptableHVector (ADVal (Flip OR.Array)) (ADVal f r y)
      , AdaptableHVector (Flip OR.Array) vals
      , AdaptableHVector (Flip OR.Array) (f r y)
@@ -259,7 +260,8 @@ crev f vals = crevDtMaybe f vals Nothing
 -- | This version additionally takes the sensitivity parameter.
 crevDt
   :: forall r y f vals advals.
-     ( AdaptableHVector (ADVal (Flip OR.Array)) advals
+     ( RankedOf f ~ Flip OR.Array
+     , AdaptableHVector (ADVal (Flip OR.Array)) advals
      , AdaptableHVector (ADVal (Flip OR.Array)) (ADVal f r y)
      , AdaptableHVector (Flip OR.Array) vals
      , AdaptableHVector (Flip OR.Array) (f r y)
@@ -270,7 +272,8 @@ crevDt f vals dt = crevDtMaybe f vals (Just dt)
 
 crevDtMaybe
   :: forall r y f vals advals.
-     ( AdaptableHVector (ADVal (Flip OR.Array)) advals
+     ( RankedOf f ~ Flip OR.Array
+     , AdaptableHVector (ADVal (Flip OR.Array)) advals
      , AdaptableHVector (ADVal (Flip OR.Array)) (ADVal f r y)
      , AdaptableHVector (Flip OR.Array) vals
      , AdaptableHVector (Flip OR.Array) (f r y)
@@ -302,7 +305,8 @@ crevDtMaybe f vals mdt =
 -- | This takes the sensitivity parameter, by convention.
 cfwd
   :: forall r y f vals advals.
-     ( AdaptableHVector (ADVal (Flip OR.Array)) advals
+     ( RankedOf f ~ Flip OR.Array
+     , AdaptableHVector (ADVal (Flip OR.Array)) advals
      , AdaptableHVector (ADVal (Flip OR.Array)) (ADVal f r y)
      , AdaptableHVector (Flip OR.Array) vals
      , AdaptableHVector (Flip OR.Array) (f r y)