[Git][ghc/ghc][master] Better error for data deriving of type synonym/family. Closes #23522

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
924a2362 by Gregory Gerasev at 2023-07-03T03:34:10-04:00
Better error for data deriving of type synonym/family. Closes #23522

- - - - -


5 changed files:

- compiler/GHC/Tc/Errors/Types.hs
- compiler/GHC/Tc/Gen/HsType.hs
- + testsuite/tests/deriving/should_fail/T23522.hs
- + testsuite/tests/deriving/should_fail/T23522.stderr
- testsuite/tests/deriving/should_fail/all.T


Changes:

=====================================
compiler/GHC/Tc/Errors/Types.hs
=====================================
@@ -1201,11 +1201,15 @@ data TcRnMessage where
   {-| TcRnIllegalClassInst is an error that occurs whenever a class instance is specified
       for a non-class.
 
+      This also includes derived instances. See the T23522 test case.
+
       Examples(s):
         type C1 a = (Show (a -> Bool))
         instance C1 Int where
 
-      Test cases: polykinds/T13267
+      Test cases:
+        polykinds/T13267
+        deriving/should_fail/T23522
   -}
   TcRnIllegalClassInst :: !(TyConFlavour TyCon) -> TcRnMessage
 


=====================================
compiler/GHC/Tc/Gen/HsType.hs
=====================================
@@ -99,7 +99,6 @@ import GHC.Tc.Utils.Instantiate ( tcInstInvisibleTyBinders, tcInstInvisibleTyBin
 import GHC.Tc.Zonk.TcType
 
 import GHC.Core.Type
-import GHC.Core.Predicate
 import GHC.Core.TyCo.Rep
 import GHC.Core.TyCo.Ppr
 
@@ -639,9 +638,15 @@ tcHsDeriv hs_ty
   = do { ty <- tcTopLHsType DerivClauseCtxt hs_ty
        ; let (tvs, pred)    = splitForAllTyCoVars ty
              (kind_args, _) = splitFunTys (typeKind pred)
-       ; case getClassPredTys_maybe pred of
-           Just (cls, tys) -> return (tvs, cls, tys, map scaledThing kind_args)
-           Nothing -> failWithTc $ TcRnIllegalDerivingItem hs_ty }
+      -- Checking that `pred` a is type class application
+       ; case splitTyConApp_maybe pred of
+          Just (tyCon, tyConArgs) ->
+            case tyConClass_maybe tyCon of
+              Just clas ->
+                return (tvs, clas, tyConArgs, map scaledThing kind_args)
+              Nothing -> failWithTc $ TcRnIllegalClassInst (tyConFlavour tyCon)
+          Nothing -> failWithTc $ TcRnIllegalDerivingItem hs_ty
+    }
 
 -- | Typecheck a deriving strategy. For most deriving strategies, this is a
 -- no-op, but for the @via@ strategy, this requires typechecking the @via@ type.


=====================================
testsuite/tests/deriving/should_fail/T23522.hs
=====================================
@@ -0,0 +1,5 @@
+module T23522 where
+
+type F x = Show x
+
+data Proposition = Proposition deriving (F)


=====================================
testsuite/tests/deriving/should_fail/T23522.stderr
=====================================
@@ -0,0 +1,5 @@
+
+T23522.hs:5:42: [GHC-53946]
+     Illegal instance for a type synonym
+      A class instance must be for a class
+     In the data declaration for ‘Proposition’


=====================================
testsuite/tests/deriving/should_fail/all.T
=====================================
@@ -87,3 +87,4 @@ test('T21087b', [extra_files(['T21087b_aux.hs','T21087b_aux.hs-boot'])], multimo
 test('T21302', normal, compile_fail, [''])
 test('T21871', normal, compile_fail, [''])
 test('T22696b', normal, compile_fail, [''])
+test('T23522', normal, compile_fail, [''])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/924a2362810d9fa27c5da212cb35fd3e357ab9d1

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/924a2362810d9fa27c5da212cb35fd3e357ab9d1
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