Make isTauTy detect higher-rank contextswip/T18127

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.

7 files changed, 91 insertions, 9 deletions

diff --git a/compiler/GHC/Core/Type.hs b/compiler/GHC/Core/Type.hs
index 1e7af2d8cf..fe6d721a05 100644
--- a/compiler/GHC/Core/Type.hs
+++ b/compiler/GHC/Core/Type.hs
@@ -1857,17 +1857,19 @@ fun_kind_arg_flags = go emptyTCvSubst
                         -- something is ill-kinded. But this can happen
                         -- when printing errors. Assume everything is Required.
 
--- @isTauTy@ tests if a type has no foralls
+-- @isTauTy@ tests if a type has no foralls or (=>)
 isTauTy :: Type -> Bool
 isTauTy ty | Just ty' <- coreView ty = isTauTy ty'
-isTauTy (TyVarTy _)           = True
-isTauTy (LitTy {})            = True
-isTauTy (TyConApp tc tys)     = all isTauTy tys && isTauTyCon tc
-isTauTy (AppTy a b)           = isTauTy a && isTauTy b
-isTauTy (FunTy _ a b)         = isTauTy a && isTauTy b
-isTauTy (ForAllTy {})         = False
-isTauTy (CastTy ty _)         = isTauTy ty
-isTauTy (CoercionTy _)        = False  -- Not sure about this
+isTauTy (TyVarTy _)       = True
+isTauTy (LitTy {})        = True
+isTauTy (TyConApp tc tys) = all isTauTy tys && isTauTyCon tc
+isTauTy (AppTy a b)       = isTauTy a && isTauTy b
+isTauTy (FunTy af a b)    = case af of
+                              InvisArg -> False                  -- e.g., Eq a => b
+                              VisArg   -> isTauTy a && isTauTy b -- e.g., a -> b
+isTauTy (ForAllTy {})     = False
+isTauTy (CastTy ty _)     = isTauTy ty
+isTauTy (CoercionTy _)    = False  -- Not sure about this
 
 {-
 %************************************************************************
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, [''])
diff --git a/testsuite/tests/typecheck/should_fail/T18127a.hs b/testsuite/tests/typecheck/should_fail/T18127a.hs
new file mode 100644
index 0000000000..48d0846841
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T18127a.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE RankNTypes #-}
+module T18127a where
+
+a :: (forall a. a) -> ()
+a = undefined
+
+b :: (Show a => a) -> ()
+b = undefined
+
+type C = forall a. a
+c :: C -> ()
+c = undefined
+
+type D a = Show a => a
+d :: D a -> ()
+d = undefined
diff --git a/testsuite/tests/typecheck/should_fail/T18127a.stderr b/testsuite/tests/typecheck/should_fail/T18127a.stderr
new file mode 100644
index 0000000000..ee354f7467
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T18127a.stderr
@@ -0,0 +1,32 @@
+
+T18127a.hs:5:5: error:
+    • Cannot instantiate unification variable ‘a1’
+      with a type involving polytypes: (forall a. a) -> ()
+        GHC doesn't yet support impredicative polymorphism
+    • In the expression: undefined
+      In an equation for ‘a’: a = undefined
+
+T18127a.hs:8:5: error:
+    • Cannot instantiate unification variable ‘a3’
+      with a type involving polytypes: (Show a => a) -> ()
+        GHC doesn't yet support impredicative polymorphism
+    • In the expression: undefined
+      In an equation for ‘b’: b = undefined
+    • Relevant bindings include
+        b :: (Show a => a) -> () (bound at T18127a.hs:8:1)
+
+T18127a.hs:12:5: error:
+    • Cannot instantiate unification variable ‘a0’
+      with a type involving polytypes: C -> ()
+        GHC doesn't yet support impredicative polymorphism
+    • In the expression: undefined
+      In an equation for ‘c’: c = undefined
+
+T18127a.hs:16:5: error:
+    • Cannot instantiate unification variable ‘a2’
+      with a type involving polytypes: D a -> ()
+        GHC doesn't yet support impredicative polymorphism
+    • In the expression: undefined
+      In an equation for ‘d’: d = undefined
+    • Relevant bindings include
+        d :: D a -> () (bound at T18127a.hs:16:1)
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 5155e76a7b..8735cead75 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -563,3 +563,4 @@ test('T17021', normal, compile_fail, [''])
 test('T17021b', normal, compile_fail, [''])
 test('T17955', normal, compile_fail, [''])
 test('T17173', normal, compile_fail, [''])
+test('T18127a', normal, compile_fail, [''])