Make isTauTy detect higher-rank contexts

Previously, `isTauTy` would only detect higher-rank `forall`s, not
higher-rank contexts, which led to some minor bugs observed
in #18127. Easily fixed by adding a case for
`(FunTy InvisArg _ _)`.

Fixes #18127.

3 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_fail/T18127b.hs b/testsuite/tests/deriving/should_fail/T18127b.hs
new file mode 100644
index 0000000000..b7aa39abf6
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T18127b.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE RankNTypes #-}
+module T18127b where
+
+import GHC.Generics
+
+data T1 = MkT1 (forall a. a) deriving (Eq, Generic)
+data T2 a = MkT2 (Show a => a) deriving (Eq, Generic)
diff --git a/testsuite/tests/deriving/should_fail/T18127b.stderr b/testsuite/tests/deriving/should_fail/T18127b.stderr
new file mode 100644
index 0000000000..9d2a289e44
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T18127b.stderr
@@ -0,0 +1,22 @@
+
+T18127b.hs:7:40: error:
+    • Can't make a derived instance of ‘Eq T1’:
+        Constructor ‘MkT1’ has a higher-rank type
+        Possible fix: use a standalone deriving declaration instead
+    • In the data declaration for ‘T1’
+
+T18127b.hs:7:44: error:
+    • Can't make a derived instance of ‘Generic T1’:
+        MkT1 must not have exotic unlifted or polymorphic arguments
+    • In the data declaration for ‘T1’
+
+T18127b.hs:8:42: error:
+    • Can't make a derived instance of ‘Eq (T2 a)’:
+        Constructor ‘MkT2’ has a higher-rank type
+        Possible fix: use a standalone deriving declaration instead
+    • In the data declaration for ‘T2’
+
+T18127b.hs:8:46: error:
+    • Can't make a derived instance of ‘Generic (T2 a)’:
+        MkT2 must not have exotic unlifted or polymorphic arguments
+    • In the data declaration for ‘T2’
diff --git a/testsuite/tests/deriving/should_fail/all.T b/testsuite/tests/deriving/should_fail/all.T
index d195a08691..4743d3530d 100644
--- a/testsuite/tests/deriving/should_fail/all.T
+++ b/testsuite/tests/deriving/should_fail/all.T
@@ -76,6 +76,7 @@ test('T15073', [extra_files(['T15073a.hs'])], multimod_compile_fail,
                ['T15073', '-v0'])
 test('T16181', normal, compile_fail, [''])
 test('T16923', normal, compile_fail, [''])
+test('T18127b', normal, compile_fail, [''])
 test('deriving-via-fail', normal, compile_fail, [''])
 test('deriving-via-fail2', normal, compile_fail, [''])
 test('deriving-via-fail3', normal, compile_fail, [''])