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

5 files changed, 33 insertions, 1 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, [''])
diff --git a/testsuite/tests/ghci/should_run/T14125a.script b/testsuite/tests/ghci/should_run/T14125a.script
new file mode 100644
index 0000000000..1667349160
--- /dev/null
+++ b/testsuite/tests/ghci/should_run/T14125a.script
@@ -0,0 +1,8 @@
+:set -XTypeFamilies
+data family Foo a
+data instance Foo Int = FooInt Int
+:kind! Foo Int
+let f (FooInt i) = i
+:info    f
+:type +v f
+:type    f
diff --git a/testsuite/tests/ghci/should_run/T14125a.stdout b/testsuite/tests/ghci/should_run/T14125a.stdout
new file mode 100644
index 0000000000..7b4e85edd3
--- /dev/null
+++ b/testsuite/tests/ghci/should_run/T14125a.stdout
@@ -0,0 +1,5 @@
+Foo Int :: *
+= Foo Int
+f :: Foo Int -> Int 	-- Defined at <interactive>:5:5
+f :: Foo Int -> Int
+f :: Foo Int -> Int
diff --git a/testsuite/tests/ghci/should_run/all.T b/testsuite/tests/ghci/should_run/all.T
index fe33685193..da20149b56 100644
--- a/testsuite/tests/ghci/should_run/all.T
+++ b/testsuite/tests/ghci/should_run/all.T
@@ -27,4 +27,5 @@ test('T11825',     just_ghci, ghci_script, ['T11825.script'])
 test('T12128',     just_ghci, ghci_script, ['T12128.script'])
 test('T12456',     just_ghci, ghci_script, ['T12456.script'])
 test('T12549',     just_ghci, ghci_script, ['T12549.script'])
-test('BinaryArray', normal, compile_and_run, [''])
\ No newline at end of file
+test('BinaryArray', normal, compile_and_run, [''])
+test('T14125a',    just_ghci, ghci_script, ['T14125a.script'])