Reject nested foralls in foreign imports (#16702)

This replaces a panic observed in #16702 with a simple error message
stating that nested `forall`s simply aren't allowed in the type
signature of a `foreign import` (at least, not at present).

Fixes #16702.

3 files changed, 54 insertions, 0 deletions

diff --git a/testsuite/tests/ffi/should_fail/T16702.hs b/testsuite/tests/ffi/should_fail/T16702.hs
new file mode 100644
index 0000000000..bad07c7120
--- /dev/null
+++ b/testsuite/tests/ffi/should_fail/T16702.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UnliftedFFITypes #-}
+{-# LANGUAGE RankNTypes #-}
+
+module T16702 where
+
+import Foreign.C.Types
+import Foreign.Ptr
+import Data.Kind (Type)
+
+foreign import ccall "math.h pow"
+  c_pow :: CDouble
+        -> forall (a :: Type). CDouble
+        -> forall (b :: Type). CDouble
+
+foreign import ccall "malloc"
+  malloc1 :: CSize -> forall a. IO (Ptr a)
+
+foreign import ccall "malloc"
+  malloc2 :: Show a => CSize -> IO (Ptr a)
+
+foreign import ccall "malloc"
+  malloc3 :: CSize -> Show a => IO (Ptr a)
diff --git a/testsuite/tests/ffi/should_fail/T16702.stderr b/testsuite/tests/ffi/should_fail/T16702.stderr
new file mode 100644
index 0000000000..2ab7c751d0
--- /dev/null
+++ b/testsuite/tests/ffi/should_fail/T16702.stderr
@@ -0,0 +1,29 @@
+
+T16702.hs:12:1: error:
+    • Unacceptable result type in foreign declaration:
+        Unexpected nested forall
+    • When checking declaration:
+        foreign import ccall safe "math.h pow" c_pow
+          :: CDouble
+             -> forall (a :: Type). CDouble -> forall (b :: Type). CDouble
+
+T16702.hs:17:1: error:
+    • Unacceptable result type in foreign declaration:
+        Unexpected nested forall
+    • When checking declaration:
+        foreign import ccall safe "malloc" malloc1
+          :: CSize -> forall a. IO (Ptr a)
+
+T16702.hs:20:1: error:
+    • Unacceptable argument type in foreign declaration:
+        ‘Show a’ cannot be marshalled in a foreign call
+    • When checking declaration:
+        foreign import ccall safe "malloc" malloc2
+          :: Show a => CSize -> IO (Ptr a)
+
+T16702.hs:23:1: error:
+    • Unacceptable argument type in foreign declaration:
+        ‘Show a’ cannot be marshalled in a foreign call
+    • When checking declaration:
+        foreign import ccall safe "malloc" malloc3
+          :: CSize -> Show a => IO (Ptr a)
diff --git a/testsuite/tests/ffi/should_fail/all.T b/testsuite/tests/ffi/should_fail/all.T
index 3bd7b5d909..38273db314 100644
--- a/testsuite/tests/ffi/should_fail/all.T
+++ b/testsuite/tests/ffi/should_fail/all.T
@@ -14,6 +14,7 @@ test('T5664', normal, compile_fail, ['-v0'])
 test('T7506', normal, compile_fail, [''])
 test('T7243', normal, compile_fail, [''])
 test('T10461', normal, compile_fail, [''])
+test('T16702', normal, compile_fail, [''])
 
 # UnsafeReenter tests implementation of an undefined behavior (calling Haskell
 # from an unsafe foreign function) and only makes sense in non-threaded way