Fix a typo in TcValidity.checkFamInstRhs

In error message generation we were using the wrong
type constructor in inst_head.  Result: the type became
ill-kinded, and that sent the compiler into a loop.

A separate patch fixes the loop. This patch fixes the
actual bug -- Trac #15473.

I also improved the "occurs more often" error message
a bit.  But it's still pretty terrible:

    * Variable ‘a’ occurs more often
      in the type family application ‘Undefined’
      than in the instance head ‘LetInterleave xs t ts is y z’

It looks like nonsense, but all becomes clear if you use
-fprint-explicit-kinds.  Really we should fix this by spotting
when invisible arguments are involved and at least suggesting
-fprint-explicit-kinds.

2 files changed, 9 insertions, 1 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T15473.stderr b/testsuite/tests/typecheck/should_compile/T15473.stderr
new file mode 100644
index 0000000000..6fdeaa115c
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T15473.stderr
@@ -0,0 +1,8 @@
+
+T15473.hs:11:3: error:
+    • Variable ‘a’ occurs more often
+        in the type family application ‘Undefined’
+        than in the instance head ‘LetInterleave xs t ts is y z’
+      (Use UndecidableInstances to permit this)
+    • In the equations for closed type family ‘LetInterleave’
+      In the type family declaration for ‘LetInterleave’
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 75f9aba433..64df3a89d4 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -647,5 +647,5 @@ test('T15431a', normal, compile, [''])
 test('T15428', normal, compile, [''])
 test('T15412', normal, compile, [''])
 test('T15141', normal, compile, [''])
-test('T15473', expect_broken(15473), compile, [''])
+test('T15473', normal, compile_fail, [''])
 test('T15499', normal, compile, [''])