Desugarer: Bring existentials in scope when substituting into record GADTs

This fixes an outright bug in which the desugarer did not bring the
existentially quantified type variables of a record GADT into `in_subst`'s
in-scope set, leading to #20278. It also addresses a minor inefficiency in
which `out_subst` was made into a substitution when a simpler `TvSubstEnv`
would suffice.

Fixes #20278.

2 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/gadt/T20278.hs b/testsuite/tests/gadt/T20278.hs
new file mode 100644
index 0000000000..436ff425cb
--- /dev/null
+++ b/testsuite/tests/gadt/T20278.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE GADTs #-}
+module T20278 where
+
+import Data.Kind
+import GHC.Exts
+
+type X1 :: TYPE rep -> Type
+data X1 a where
+  MkX1 :: { fld1a :: a, fld1b :: Int } -> X1 a
+
+upd1 :: forall rep (a :: TYPE rep). X1 a -> X1 a
+upd1 x = x { fld1b = 3 }
+
+type X2 :: Type -> Type
+data X2 a where
+  MkX2 :: { fld2a :: b, fld2b :: Int } -> X2 (Maybe b)
+
+upd2 :: X2 a -> X2 a
+upd2 x = x { fld2b = 3 }
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index 91ba74b722..9179a40288 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -120,4 +120,5 @@ test('T15009', normal, compile, [''])
 test('T15558', normal, compile, [''])
 test('T16427', normal, compile_fail, [''])
 test('T18191', normal, compile_fail, [''])
+test('T20278', normal, compile, [''])
 test('SynDataRec', normal, compile, [''])