8 files changed, 75 insertions, 14 deletions

diff --git a/compiler/GHC/Tc/Validity.hs b/compiler/GHC/Tc/Validity.hs
index cad2ea1796..8be1944651 100644
--- a/compiler/GHC/Tc/Validity.hs
+++ b/compiler/GHC/Tc/Validity.hs
@@ -1385,21 +1385,24 @@ check_special_inst_head dflags is_boot is_sig ctxt clas cls_args
   , not is_boot
   = failWithTc (TcRnAbstractClassInst clas)
 
-  -- For Typeable, don't complain about instances for
-  -- standalone deriving; they are no-ops, and we warn about
-  -- it in GHC.Tc.Deriv.deriveStandalone.
+  -- Complain about hand-written instances of built-in classes
+  -- Typeable, KnownNat, KnownSymbol, Coercible, HasField.
+
+  -- Disallow hand-written Typeable instances, except that we
+  -- allow a standalone deriving declaration: they are no-ops,
+  -- and we warn about them in GHC.Tc.Deriv.deriveStandalone.
   | clas_nm == typeableClassName
   , not is_sig
     -- Note [Instances of built-in classes in signature files]
   , hand_written_bindings
   = failWithTc $ TcRnSpecialClassInst clas False
 
-  -- Handwritten instances of KnownNat/KnownSymbol
-  -- are forbidden outside of signature files (#12837)
-  | clas_nm `elem` [ knownNatClassName, knownSymbolClassName ]
-  , not is_sig
+  -- Handwritten instances of KnownNat/KnownChar/KnownSymbol
+  -- are forbidden outside of signature files (#12837).
+  -- Derived instances are forbidden completely (#21087).
+  | clas_nm `elem` [ knownNatClassName, knownSymbolClassName, knownCharClassName ]
+  , (not is_sig && hand_written_bindings) || derived_instance
     -- Note [Instances of built-in classes in signature files]
-  , hand_written_bindings
   = failWithTc $ TcRnSpecialClassInst clas False
 
   -- For the most part we don't allow
@@ -1438,12 +1441,18 @@ check_special_inst_head dflags is_boot is_sig ctxt clas cls_args
     ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
 
     hand_written_bindings
-        = case ctxt of
-            InstDeclCtxt stand_alone -> not stand_alone
-            SpecInstCtxt             -> False
-            DerivClauseCtxt          -> False
-            SigmaCtxt                -> False
-            _                        -> True
+      = case ctxt of
+          InstDeclCtxt standalone -> not standalone
+          SpecInstCtxt            -> False
+          DerivClauseCtxt         -> False
+          SigmaCtxt               -> False
+          _                       -> True
+
+    derived_instance
+      = case ctxt of
+            InstDeclCtxt standalone -> standalone
+            DerivClauseCtxt         -> True
+            _                       -> False
 
     check_h98_arg_shape = case ctxt of
                             SpecInstCtxt    -> False
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', ''])