Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: b757727a by Vladislav Zavialov at 2026-08-14T15:17:27-04:00 Fix tcLookupId panic with RequiredTypeArguments and PatternSynonyms (#27586) The arguments declared on the left-hand side of a pattern synonym are looked up as term variables bound by its right-hand side. Prior to this patch, that lookup panicked with RequiredTypeArguments: data T a where MkT :: forall a -> T a pattern P :: Int -> T Int pattern P x = MkT x On the RHS, `x` looks like a term argument, so the renamer binds it in the term namespace. Only during type checking does it turn out to be a type variable, so the lookup on the LHS finds an ATyVar rather than an ATcId. As the lookup was done with tcLookupId, it resulted in a panic. Now the arguments are looked up with tcLookupPatSynArg, which reports an illegal term-level use of `x`, just as an ordinary function definition `f (MkT x) = x` does. Test cases: T27586a T27586b T27586c Assisted-by: Claude Opus 5 - - - - - 9 changed files: - + changelog.d/T27586 - compiler/GHC/Tc/TyCl/PatSyn.hs - + testsuite/tests/vdq-rta/should_fail/T27586a.hs - + testsuite/tests/vdq-rta/should_fail/T27586a.stderr - + testsuite/tests/vdq-rta/should_fail/T27586b.hs - + testsuite/tests/vdq-rta/should_fail/T27586b.stderr - + testsuite/tests/vdq-rta/should_fail/T27586c.hs - + testsuite/tests/vdq-rta/should_fail/T27586c.stderr - testsuite/tests/vdq-rta/should_fail/all.T Changes: ===================================== changelog.d/T27586 ===================================== @@ -0,0 +1,9 @@ +section: compiler +issues: #27586 +mrs: !16440 +synopsis: + Fix a panic on a required type argument in a pattern synonym RHS +description: + An argument of a pattern synonym that is matched against a required type + argument in the right-hand side no longer causes a panic; it is reported as + an illegal term-level use of a type variable. ===================================== compiler/GHC/Tc/TyCl/PatSyn.hs ===================================== @@ -137,7 +137,7 @@ tcInferPatSynDecl (PSB { psb_id = lname@(L _ name), psb_args = details ; (tclvl, wanted, ((lpat', args), pat_ty)) <- pushLevelAndCaptureConstraints $ tcInferPat FRRPatSynArg PatSynCtx lpat $ - mapM tcLookupId arg_names + mapM tcLookupPatSynArg arg_names ; let (ex_tvs, prov_dicts) = tcCollectEx lpat' @@ -472,7 +472,7 @@ tcCheckPatSynDecl psb@PSB{ psb_id = lname@(L _ name), psb_args = details -- location to x's binding site in lpat, namely the 'x' in Just (x,True). -- Else the error message location is wherever tcCheckPat finished, -- namely the right-hand corner of the pattern - do { arg_id <- tcLookupId arg_name + do { arg_id <- tcLookupPatSynArg arg_name ; wrap <- tcSubTypeSigma (OccurrenceOf (idName arg_id)) GenSigCtxt (idType arg_id) @@ -645,6 +645,19 @@ collectPatSynArgInfo details = InfixCon _ name1 name2 -> (map unLoc [name1, name2], True) RecCon _ names -> (map (unLoc . recordPatSynPatVar) names, False) +-- | Look up the 'Id' bound by the pattern for a declared argument of a pattern +-- synonym. With @RequiredTypeArguments@ the argument may turn out to be a type +-- variable, e.g. @pattern P x = MkT x@ where the argument of @MkT@ is a required +-- type argument; then we report an illegal term-level use of @x@ (#27586). +tcLookupPatSynArg :: Name -> TcM Id +tcLookupPatSynArg arg_name + = do { thing <- tcLookup arg_name + ; case thing of + ATcId { tct_id = id } -> return id + AGlobal (AnId id) -> return id + ATyVar {} -> failIllegalTyVar (noUserRdr arg_name) + _ -> pprPanic "tcLookupPatSynArg" (ppr arg_name) } + wrongNumberOfParmsErr :: Name -> Arity -> Arity -> TcM a wrongNumberOfParmsErr name decl_arity missing = failWithTc $ TcRnPatSynArityMismatch name decl_arity missing ===================================== testsuite/tests/vdq-rta/should_fail/T27586a.hs ===================================== @@ -0,0 +1,9 @@ +{-# LANGUAGE GADTs, RequiredTypeArguments, PatternSynonyms #-} + +module T27586a where + +data T a where + MkT :: forall a -> T a + +pattern P :: Int -> T Int +pattern P x = MkT x ===================================== testsuite/tests/vdq-rta/should_fail/T27586a.stderr ===================================== @@ -0,0 +1,5 @@ +T27586a.hs:9:19: error: [GHC-01928] + • Illegal term-level use of the type variable ‘x’ + • bound at T27586a.hs:9:19 + • In the declaration for pattern synonym ‘P’ + ===================================== testsuite/tests/vdq-rta/should_fail/T27586b.hs ===================================== @@ -0,0 +1,8 @@ +{-# LANGUAGE GADTs, RequiredTypeArguments, PatternSynonyms #-} + +module T27586b where + +data T a where + MkT :: forall a -> T a + +pattern P x = MkT x ===================================== testsuite/tests/vdq-rta/should_fail/T27586b.stderr ===================================== @@ -0,0 +1,5 @@ +T27586b.hs:8:15: error: [GHC-01928] + • Illegal term-level use of the type variable ‘x’ + • bound at T27586b.hs:8:19 + • In the declaration for pattern synonym ‘P’ + ===================================== testsuite/tests/vdq-rta/should_fail/T27586c.hs ===================================== @@ -0,0 +1,9 @@ +{-# LANGUAGE GADTs, RequiredTypeArguments, PatternSynonyms #-} + +module T27586c where + +data T a where + MkT :: forall a -> T a + +pattern P :: Int -> T Int +pattern P x <- MkT x ===================================== testsuite/tests/vdq-rta/should_fail/T27586c.stderr ===================================== @@ -0,0 +1,5 @@ +T27586c.hs:9:20: error: [GHC-01928] + • Illegal term-level use of the type variable ‘x’ + • bound at T27586c.hs:9:20 + • In the declaration for pattern synonym ‘P’ + ===================================== testsuite/tests/vdq-rta/should_fail/all.T ===================================== @@ -35,3 +35,6 @@ test('T25127_fail_arity', normal, compile_fail, ['']) test('T27440e', normal, compile_fail, ['']) test('T27583f', normal, compile_fail, ['']) +test('T27586a', normal, compile_fail, ['']) +test('T27586b', normal, compile_fail, ['']) +test('T27586c', normal, compile_fail, ['']) View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b757727a78613e7437a713058c24b94e... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b757727a78613e7437a713058c24b94e... 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