Use isReflexiveCo when building an MCorecionwip/T19815

This patch just changes `isReflCo` to `isReflexiveCo` when we build an
MCoercion, in `mkMCo`.  It turns out that this makes a gigantic
difference to test T18223.

I also changed the name from `coToMCo` to `mkMCo`, which is more
consistent with how we name smart constructors.

2 files changed, 17 insertions, 13 deletions

diff --git a/compiler/GHC/Core/Coercion.hs b/compiler/GHC/Core/Coercion.hs
index b364091958..ccf5ef1f11 100644
--- a/compiler/GHC/Core/Coercion.hs
+++ b/compiler/GHC/Core/Coercion.hs
@@ -72,7 +72,7 @@ module GHC.Core.Coercion (
         isReflCoVar_maybe, isGReflMCo, mkGReflLeftMCo, mkGReflRightMCo,
         mkCoherenceRightMCo,
 
-        coToMCo, mkTransMCo, mkTransMCoL, mkCastTyMCo, mkSymMCo, isReflMCo,
+        mkMCo, mkTransMCo, mkTransMCoL, mkTransMCoR, mkCastTyMCo, mkSymMCo, isReflMCo,
 
         -- ** Coercion variables
         mkCoVar, isCoVar, coVarName, setCoVarName, setCoVarUnique,
@@ -305,11 +305,13 @@ tidyCoAxBndrsForUser init_env tcvs
 *                                                                      *
 ********************************************************************* -}
 
-coToMCo :: Coercion -> MCoercion
--- Convert a coercion to a MCoercion,
--- It's not clear whether or not isReflexiveCo would be better here
-coToMCo co | isReflCo co = MRefl
-           | otherwise   = MCo co
+mkMCo :: Coercion -> MCoercion
+-- Convert a coercion to a MCoercion
+-- I'm using isReflexiveCo to check whether the coercion is reflexive,
+-- because it makes a gigantic difference in T18223, and does little
+-- harm otherwise (#19815)
+mkMCo co | isReflexiveCo co = MRefl
+         | otherwise        = MCo co
 
 -- | Tests if this MCoercion is obviously generalized reflexive
 -- Guaranteed to work very quickly.
@@ -329,11 +331,15 @@ mkGReflCo r ty mco
 mkTransMCo :: MCoercion -> MCoercion -> MCoercion
 mkTransMCo MRefl     co2       = co2
 mkTransMCo co1       MRefl     = co1
-mkTransMCo (MCo co1) (MCo co2) = MCo (mkTransCo co1 co2)
+mkTransMCo (MCo co1) (MCo co2) = mkMCo (mkTransCo co1 co2)
 
 mkTransMCoL :: MCoercion -> Coercion -> MCoercion
 mkTransMCoL MRefl     co2 = MCo co2
-mkTransMCoL (MCo co1) co2 = MCo (mkTransCo co1 co2)
+mkTransMCoL (MCo co1) co2 = mkMCo (mkTransCo co1 co2)
+
+mkTransMCoR :: Coercion -> MCoercion -> MCoercion
+mkTransMCoR co1 MRefl     = MCo co1
+mkTransMCoR co1 (MCo co2) = mkMCo (mkTransCo co1 co2)
 
 -- | Get the reverse of an 'MCoercion'
 mkSymMCo :: MCoercion -> MCoercion
diff --git a/compiler/GHC/Core/Opt/Arity.hs b/compiler/GHC/Core/Opt/Arity.hs
index 36a2535c09..aa0b936058 100644
--- a/compiler/GHC/Core/Opt/Arity.hs
+++ b/compiler/GHC/Core/Opt/Arity.hs
@@ -1672,11 +1672,9 @@ pushCoValArg co
               -- then co1 :: tyL1 ~ tyR1
               --      co2 :: tyL2 ~ tyR2
   = ASSERT2( isFunTy tyR, ppr co $$ ppr arg )
-    Just (coToMCo (mkSymCo co1), coToMCo co2)
-    -- Critically, coToMCo to checks for ReflCo; the whole coercion may not
-    -- be reflexive, but either of its components might be
-    -- We could use isReflexiveCo, but it's not clear if the benefit
-    -- is worth the cost, and it makes no difference in #18223
+    Just (mkMCo (mkSymCo co1), mkMCo co2)
+    -- Critically, mkMCo to checks for isReflexiveCo; the whole coercion
+    -- may not be reflexive, but either of its components might be
 
   | otherwise
   = Nothing