Broaden the in-scope sets for liftEnvSubst and composeTCvSubst

This patch fixes two distinct (but closely related) buglets that were uncovered
in #22235:

* `liftEnvSubst` used an empty in-scope set, which was not wide enough to cover
  the variables in the range of the substitution. This patch fixes this by
  populating the in-scope set from the free variables in the range of the
  substitution.
* `composeTCvSubst` applied the first substitution argument to the range of the
  second substitution argument, but the first substitution's in-scope set was
  not wide enough to cover the range of the second substutition. We similarly
  fix this issue in this patch by widening the first substitution's in-scope set
  before applying it.

Fixes #22235.

2 files changed, 24 insertions, 0 deletions

diff --git a/testsuite/tests/gadt/T22235.hs b/testsuite/tests/gadt/T22235.hs
new file mode 100644
index 0000000000..0f4161db72
--- /dev/null
+++ b/testsuite/tests/gadt/T22235.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+module T22235 (f) where
+
+import Data.Kind (Type)
+import Data.Type.Equality ((:~:)(..))
+
+f :: ST x -> ST y -> x :~: y
+f st1@SMkT st2@SMkT = method st1 st2
+
+type T :: Type -> Type
+data T a where
+  MkT :: T Int
+
+type ST :: T a -> Type
+data ST (t :: T a) where
+  SMkT :: ST MkT
+
+class C f where
+  method :: f a -> f b -> a :~: b
+
+instance C ST where
+  method SMkT SMkT = Refl
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index ce30d570f3..1b32762525 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -125,3 +125,4 @@ test('T20278', normal, compile, [''])
 test('SynDataRec', normal, compile, [''])
 test('T20485', normal, compile, [''])
 test('T20485a', normal, compile, [''])
+test('T22235', normal, compile, [''])