Fix the free-var test in validDerivPred

The free-var test (now documented as (VD3)) was too narrow,
affecting only class predicates.  #21302 demonstrated that
this wasn't enough!

Fixes #21302.

Co-authored-by: Ryan Scott <ryan.gl.scott@gmail.com>

3 files changed, 32 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_fail/T21302.hs b/testsuite/tests/deriving/should_fail/T21302.hs
new file mode 100644
index 0000000000..16e7cf320d
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21302.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE UndecidableInstances, TypeFamilies #-}
+
+module T21302 where
+
+data BoxAssocDouble = BoxAssocDouble (BoxAssoc Int)
+  deriving (Eq)
+
+type family Assoc a
+
+data BoxAssoc a = BoxAssoc (Assoc a)
+
+deriving instance c Eq a => Eq (BoxAssoc a)
diff --git a/testsuite/tests/deriving/should_fail/T21302.stderr b/testsuite/tests/deriving/should_fail/T21302.stderr
new file mode 100644
index 0000000000..46910cbf4f
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21302.stderr
@@ -0,0 +1,19 @@
+
+T21302.hs:6:13: error:
+    • Could not solve: (c0 Eq Int)
+        arising from the first field of ‘BoxAssocDouble’
+          (type ‘BoxAssoc Int’)
+    • When deriving the instance for (Eq BoxAssocDouble)
+
+T21302.hs:12:19: error:
+    • Could not deduce (c0 Eq a)
+      from the context: c Eq a
+        bound by a stand-alone deriving instance declaration:
+                   forall (c :: (* -> Constraint) -> * -> Constraint) a.
+                   c Eq a =>
+                   Eq (BoxAssoc a)
+        at T21302.hs:12:19-43
+    • In the ambiguity check for a stand-alone deriving instance declaration
+      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
+      In the stand-alone deriving instance for
+        ‘c Eq a => Eq (BoxAssoc a)’
diff --git a/testsuite/tests/deriving/should_fail/all.T b/testsuite/tests/deriving/should_fail/all.T
index 61d74b72b4..2b2f2af562 100644
--- a/testsuite/tests/deriving/should_fail/all.T
+++ b/testsuite/tests/deriving/should_fail/all.T
@@ -82,3 +82,4 @@ test('deriving-via-fail4', normal, compile_fail, [''])
 test('deriving-via-fail5', normal, compile_fail, [''])
 test('T21087', normal, compile_fail, [''])
 test('T21087b', [extra_files(['T21087b_aux.hs','T21087b_aux.hs-boot'])], multimod_compile_fail, ['T21087b', ''])
+test('T21302', normal, compile_fail, [''])