Forbid standalone instances for built-in classes

  `check_special_inst_head` includes logic that disallows hand-written
  instances for built-in classes such as Typeable, KnownNat
  and KnownSymbol.
  However, it also allowed standalone deriving declarations. This was
  because we do want to allow standalone deriving instances with
  Typeable as they are harmless, but we certainly don't want to allow
  instances for e.g. KnownNat.

  This patch ensures that we don't allow derived instances for
  KnownNat, KnownSymbol (and also KnownChar, which was previously
  omitted entirely).

  Fixes #21087

7 files changed, 52 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_fail/T21087.hs b/testsuite/tests/deriving/should_fail/T21087.hs
new file mode 100644
index 0000000000..730dbac153
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21087.hs
@@ -0,0 +1,11 @@
+{-# language DataKinds #-}
+{-# language DerivingVia #-}
+{-# language TypeFamilies #-}
+
+module T21087 where
+
+import GHC.TypeLits
+
+data family Z :: k
+
+deriving via 0 instance KnownNat Z
diff --git a/testsuite/tests/deriving/should_fail/T21087.stderr b/testsuite/tests/deriving/should_fail/T21087.stderr
new file mode 100644
index 0000000000..8e40f6e641
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21087.stderr
@@ -0,0 +1,4 @@
+
+T21087.hs:11:25: error:
+    • Class ‘KnownNat’ does not support user-specified instances.
+    • In the stand-alone deriving instance for ‘KnownNat Z’
diff --git a/testsuite/tests/deriving/should_fail/T21087b.hs b/testsuite/tests/deriving/should_fail/T21087b.hs
new file mode 100644
index 0000000000..522c1603a7
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21087b.hs
@@ -0,0 +1,3 @@
+module T21087b where
+
+import {-# SOURCE #-} T21087b_aux
diff --git a/testsuite/tests/deriving/should_fail/T21087b.stderr b/testsuite/tests/deriving/should_fail/T21087b.stderr
new file mode 100644
index 0000000000..4caee4928e
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21087b.stderr
@@ -0,0 +1,10 @@
+[1 of 3] Compiling T21087b_aux[boot] ( T21087b_aux.hs-boot, T21087b_aux.o-boot )
+
+T21087b_aux.hs-boot:11:25: error:
+    • Class ‘KnownNat’ does not support user-specified instances.
+    • In the stand-alone deriving instance for ‘KnownNat Z’
+[3 of 3] Compiling T21087b_aux      ( T21087b_aux.hs, T21087b_aux.o )
+
+T21087b_aux.hs:11:25: error:
+    • Class ‘KnownNat’ does not support user-specified instances.
+    • In the stand-alone deriving instance for ‘KnownNat Z’
diff --git a/testsuite/tests/deriving/should_fail/T21087b_aux.hs b/testsuite/tests/deriving/should_fail/T21087b_aux.hs
new file mode 100644
index 0000000000..2cccf704ea
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21087b_aux.hs
@@ -0,0 +1,11 @@
+{-# language DataKinds #-}
+{-# language DerivingVia #-}
+{-# language TypeFamilies #-}
+
+module T21087b_aux where
+
+import GHC.TypeLits
+
+data family Z :: k
+
+deriving via 0 instance KnownNat Z
diff --git a/testsuite/tests/deriving/should_fail/T21087b_aux.hs-boot b/testsuite/tests/deriving/should_fail/T21087b_aux.hs-boot
new file mode 100644
index 0000000000..2cccf704ea
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21087b_aux.hs-boot
@@ -0,0 +1,11 @@
+{-# language DataKinds #-}
+{-# language DerivingVia #-}
+{-# language TypeFamilies #-}
+
+module T21087b_aux where
+
+import GHC.TypeLits
+
+data family Z :: k
+
+deriving via 0 instance KnownNat Z
diff --git a/testsuite/tests/deriving/should_fail/all.T b/testsuite/tests/deriving/should_fail/all.T
index 70bd0f9dbe..61d74b72b4 100644
--- a/testsuite/tests/deriving/should_fail/all.T
+++ b/testsuite/tests/deriving/should_fail/all.T
@@ -80,3 +80,5 @@ test('deriving-via-fail2', normal, compile_fail, [''])
 test('deriving-via-fail3', normal, compile_fail, [''])
 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', ''])