Fix #14125 by normalizing data family instances more aggressively

Summary:
Commit 3540d1e1a23926ce0a8a6ae83a36f5f6b2497ccf inadvertently broke
the ability for newtype instances to be used as marshallable types in FFI
declarations. The reason is a bit silly: an extra check was added for type
synonyms with no type families on the RHS in `normalise_tc_app`, but this check
would only skip over type families, not //data// families, since the predicate
being used was `not . isTypeFamilyCon`.

The fix is simple: just use `not . isFamilyCon` instead so that data families
are also skipped by this check.

Test Plan: make test TEST=T14125

Reviewers: goldfire, simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #14125

Differential Revision: https://phabricator.haskell.org/D3865

2 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/ffi/should_compile/T14125.hs b/testsuite/tests/ffi/should_compile/T14125.hs
new file mode 100644
index 0000000000..daf236d5ca
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T14125.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+{-# LANGUAGE TypeFamilies #-}
+module T14125 where
+
+import Foreign.C.String
+import Foreign.C.Types
+
+data UnixReturn
+
+data family IOErrno a
+newtype instance IOErrno UnixReturn = UnixErrno CInt
+
+foreign import ccall unsafe "string.h"
+  strerror :: IOErrno UnixReturn -> IO CString
+
+foreign import ccall unsafe "HsBase.h __hscore_get_errno"
+  get_errno :: IO (IOErrno UnixReturn)
diff --git a/testsuite/tests/ffi/should_compile/all.T b/testsuite/tests/ffi/should_compile/all.T
index 18192d49ff..0f2f3901c4 100644
--- a/testsuite/tests/ffi/should_compile/all.T
+++ b/testsuite/tests/ffi/should_compile/all.T
@@ -31,3 +31,4 @@ test('cc015', normal, compile, [''])
 test('cc016', normal, compile, [''])
 test('T10460', normal, compile, [''])
 test('T11983', [omit_ways(['ghci'])], compile, ['T11983.c'])
+test('T14125', normal, compile, [''])