From 21f17d06aa5c33e639f1b0d37b4bf888b494c441 Mon Sep 17 00:00:00 2001
From: Simon Peyton Jones <simonpj@microsoft.com>
Date: Tue, 13 May 2014 13:17:19 +0100
Subject: [PATCH] Fix invariant in mkAppCoFlexible
mkAppCoFlexible was breaking the invariant that the head of a TyConAppCo cannot
be a type synonym. This small patch fixes it.
---
compiler/types/Coercion.lhs | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/compiler/types/Coercion.lhs b/compiler/types/Coercion.lhs
index 195bc4cbf2b..a8f9a2ee599 100644
--- a/compiler/types/Coercion.lhs
+++ b/compiler/types/Coercion.lhs
@@ -948,7 +948,9 @@ mkAppCo co1 co2 = mkAppCoFlexible co1 Nominal co2
mkAppCoFlexible :: Coercion -> Role -> Coercion -> Coercion
mkAppCoFlexible (Refl r ty1) _ (Refl _ ty2)
= Refl r (mkAppTy ty1 ty2)
-mkAppCoFlexible (Refl r (TyConApp tc tys)) r2 co2
+mkAppCoFlexible (Refl r ty1) r2 co2
+ | Just (tc, tys) <- splitTyConApp_maybe ty1
+ -- Expand type synonyms; a TyConAppCo can't have a type synonym (Trac #9102)
= TyConAppCo r tc (zip_roles (tyConRolesX r tc) tys)
where
zip_roles (r1:_) [] = [downgradeRole r1 r2 co2]
--
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