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.

1 files changed, 23 insertions, 3 deletions

diff --git a/docs/users_guide/ffi-chap.rst b/docs/users_guide/ffi-chap.rst
index b90b48a672..13ea352c8d 100644
--- a/docs/users_guide/ffi-chap.rst
+++ b/docs/users_guide/ffi-chap.rst
@@ -14,9 +14,10 @@ Foreign function interface (FFI)
 
     Allow use of the Haskell foreign function interface.
 
-GHC (mostly) conforms to the Haskell Foreign Function Interface, whose
-definition is part of the Haskell Report on
-`http://www.haskell.org/ <http://www.haskell.org/>`__.
+GHC (mostly) conforms to the Haskell Foreign Function Interface as specified
+in the Haskell Report. Refer to the `relevant chapter
+<https://www.haskell.org/onlinereport/haskell2010/haskellch8.html>_`
+of the Haskell Report for more details.
 
 FFI support is enabled by default, but can be enabled or disabled
 explicitly with the :extension:`ForeignFunctionInterface` flag.
@@ -102,6 +103,25 @@ OK: ::
        foreign import foo :: Int -> MyIO Int
        foreign import "dynamic" baz :: (Int -> MyIO Int) -> CInt -> MyIO Int
 
+.. _ffi-foralls:
+
+Explicit ``forall``s in foreign types
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The type variables in the type of a foreign declaration may be quantified with
+an explicit ``forall`` by using the :extension:`ExplicitForAll` language
+extension, as in the following example: ::
+
+    {-# LANGUAGE ExplicitForAll #-}
+    foreign import ccall "mmap" c_mmap :: forall a. CSize -> IO (Ptr a)
+
+Note that an explicit ``forall`` must appear at the front of the type signature
+and is not permitted to appear nested within the type, as in the following
+(erroneous) examples: ::
+
+    foreign import ccall "mmap" c_mmap' :: CSize -> forall a. IO (Ptr a)
+    foreign import ccall quux :: (forall a. Ptr a) -> IO ()
+
 .. _ffi-prim:
 
 Primitive imports