Enable ImpredicativeTypes internally when typechecking selector bindings

This is necessary for certain record selectors with higher-rank
types, such as the examples in #18005. See
`Note [Impredicative record selectors]` in `TcTyDecls`.

Fixes #18005.

2 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T18005.hs b/testsuite/tests/typecheck/should_compile/T18005.hs
new file mode 100644
index 0000000000..9b4ddbd366
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T18005.hs
@@ -0,0 +1,30 @@
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ViewPatterns #-}
+module T18005 where
+
+type S1 = Int -> (forall a. a -> a) -> Int
+
+data T1a = MkT1a {unT1a :: S1}
+         | Dummy1
+
+newtype T1b = MkT1b S1
+
+unT1b' :: T1b -> S1
+unT1b' (MkT1b x) = x
+
+pattern MkT1b' :: S1 -> T1b
+pattern MkT1b' {unT1b} <- (unT1b' -> unT1b)
+
+type S2 = Int -> forall a. a -> a
+
+data T2a = MkT2a {unT2a :: S2}
+         | Dummy2
+
+newtype T2b = MkT2b S2
+
+unT2b' :: T2b -> S2
+unT2b' (MkT2b x) = x
+
+pattern MkT2b' :: S2 -> T2b
+pattern MkT2b' {unT2b} <- (unT2b' -> unT2b)
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index dd416ad2de..9b60c6cfb0 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -701,3 +701,4 @@ test('T17710', normal, compile, [''])
 test('T17792', normal, compile, [''])
 test('T17024', normal, compile, [''])
 test('T17021a', normal, compile, [''])
+test('T18005', normal, compile, [''])