Fix lexically-scoped type variables

Trac #13881 showed that our handling of lexically scoped type
variables was way off when we bring into scope a name 'y' for
a pre-existing type variable 'a', perhaps with an entirely
different name.

This patch fixes it; see TcHsType
  Note [Pattern signature binders]

2 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T13881.hs b/testsuite/tests/typecheck/should_compile/T13881.hs
new file mode 100644
index 0000000000..5f79f99ef8
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T13881.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+module T13881 where
+
+data family Sing (a :: k)
+
+data instance Sing (z :: [a]) where
+  SNil  :: Sing '[]
+  SCons :: Sing x -> Sing xs -> Sing (x ': xs)
+
+fl :: forall (l :: [a]). Sing l -> Sing l
+fl (SNil :: Sing (l :: [y])) = SNil
+fl (SCons x xs)              = SCons x xs
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 2fc0241fb7..d6aaef53d2 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -566,3 +566,4 @@ test('T13804', normal, compile, [''])
 test('T13822', normal, compile, [''])
 test('T13871', normal, compile, [''])
 test('T13879', normal, compile, [''])
+test('T13881', normal, compile, [''])