Adding test case for polytype decomposition in the constraint solver.

1 files changed, 32 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/PolytypeDecomp.hs b/testsuite/tests/typecheck/should_compile/PolytypeDecomp.hs
new file mode 100644
index 0000000000..69e4fb31c7
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/PolytypeDecomp.hs
@@ -0,0 +1,32 @@
+{-# LANGUAGE TypeFamilies, LiberalTypeSynonyms, ImpredicativeTypes #-}
+module PolyTypeDecomp where 
+
+
+{- The purpose of this test is to check if decomposition of wanted 
+   equalities in the /constraint solver/ (vs. the unifier) works properly.
+   Unfortunately most equalities between polymorphic types are converted to 
+   implication constraints early on in the unifier, so we have to make things
+   a bit more convoluted by introducing the myLength function. The wanted 
+   constraints we get for this program are:
+      [forall a. Maybe a] ~ Id alpha
+      [forall a. F [a]]   ~ Id alpha 
+   Which, /after reactions/ should create a fresh implication: 
+      forall a. Maybe a ~ F [a]
+   that is perfectly soluble.
+-}
+ 
+type family F a
+type instance F [a] = Maybe a 
+
+type family Id a 
+type instance Id a = a
+
+f :: [forall a. F [a]]
+f = undefined
+
+
+g :: [forall a. Maybe a] -> Int
+g x = myLength [x,f]
+
+myLength :: [Id a] -> Int 
+myLength = undefined