Fix #16002 by moving a validity check to the renamer

Summary:
The validity check which rejected things like:

```lang=haskell
type family B x where
  A x = x
```

Used to live in the typechecker. But it turns out that this validity
check was //only// being run on closed type families without CUSKs!
This meant that GHC would silently accept something like this:

```lang=haskell
type family B (x :: *) :: * where
  A x = x
```

This patch fixes the issue by moving this validity check to the
renamer, where we can be sure that the check will //always// be run.

Test Plan: make test TEST=T16002

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: goldfire, rwbarton, carter

GHC Trac Issues: #16002

Differential Revision: https://phabricator.haskell.org/D5420

3 files changed, 13 insertions, 0 deletions

diff --git a/testsuite/tests/rename/should_fail/T16002.hs b/testsuite/tests/rename/should_fail/T16002.hs
new file mode 100644
index 0000000000..00aadf14dd
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T16002.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TypeFamilies #-}
+module T16002 where
+
+data A
+type family B (x :: *) :: * where
+  A x = x
diff --git a/testsuite/tests/rename/should_fail/T16002.stderr b/testsuite/tests/rename/should_fail/T16002.stderr
new file mode 100644
index 0000000000..98db6f99b6
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T16002.stderr
@@ -0,0 +1,6 @@
+
+T16002.hs:6:3: error:
+    Mismatched type name in type family instance.
+      Expected: B
+        Actual: A
+    In the declaration for type family ‘B’
diff --git a/testsuite/tests/rename/should_fail/all.T b/testsuite/tests/rename/should_fail/all.T
index ba6975483a..56934266d2 100644
--- a/testsuite/tests/rename/should_fail/all.T
+++ b/testsuite/tests/rename/should_fail/all.T
@@ -142,5 +142,6 @@ test('T15607', normal, compile_fail, [''])
 test('T15611a', normal, compile_fail, [''])
 test('T15611b', normal, ghci_script, ['T15611b.script'])
 test('T15828', normal, compile_fail, [''])
+test('T16002', normal, compile_fail, [''])
 
 test('ExplicitForAllRules2', normal, compile_fail, [''])