Simon Peyton Jones pushed to branch wip/T26255 at Glasgow Haskell Compiler / GHC Commits: 745cae95 by Simon Peyton Jones at 2025-08-23T23:31:48+01:00 More wibbles - - - - - 3 changed files: - compiler/GHC/Tc/Errors.hs - testsuite/tests/typecheck/should_fail/T18851.hs - testsuite/tests/typecheck/should_fail/UnliftedNewtypesFamilyKindFail2.stderr Changes: ===================================== compiler/GHC/Tc/Errors.hs ===================================== @@ -443,7 +443,6 @@ ignoreConstraint :: Ct -> Bool ignoreConstraint ct = case ctOrigin ct of AssocFamPatOrigin -> True -- See (CIG1) - WantedSuperclassOrigin {} -> True -- See (CIG2) _ -> False -- | Makes an error item from a constraint, calculating whether or not @@ -540,7 +539,7 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics ; when (null simples) $ reportMultiplicityCoercionErrs ctxt_for_insols mult_co_errs -- See Note [Suppressing confusing errors] - ; let (suppressed_items, items0) = partition suppress tidy_items + ; let (suppressed_items, items0) = partition suppressItem tidy_items ; traceTc "reportWanteds suppressed:" (ppr suppressed_items) ; (ctxt1, items1) <- tryReporters ctxt_for_insols report1 items0 @@ -548,7 +547,7 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics -- any of the first batch failed, or if the enclosing context -- says to suppress ; let ctxt2 = ctxt1 { cec_suppress = cec_suppress ctxt || cec_suppress ctxt1 } - ; (ctxt3, leftovers) <- tryReporters ctxt2 report2 items1 + ; (_, leftovers) <- tryReporters ctxt2 report2 items1 ; massertPpr (null leftovers) (text "The following unsolved Wanted constraints \ \have not been reported to the user:" @@ -559,12 +558,15 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics -- wanted insoluble here; but do suppress inner insolubles -- if there's a *given* insoluble here (= inaccessible code) - -- Only now, if there are no errors, do we report suppressed ones - -- See Note [Suppressing confusing errors] - -- We don't need to update the context further because of the - -- whenNoErrs guard + -- Only now, if there are no errors, do we report suppressed ones + -- See Note [Suppressing confusing errors]. We don't need to update + -- the context further because of the whenNoErrs guard ; whenNoErrs $ - do { (_, more_leftovers) <- tryReporters ctxt3 report3 suppressed_items + do { (_, more_leftovers) <- tryReporters ctxt_for_insols (report1++report2) + suppressed_items + -- ctxt_for_insols: the suppressed errors can be Int~Bool, which + -- will have made the incoming `ctxt` be True; don't make that suppress + -- the Int~Bool error! ; massertPpr (null more_leftovers) (ppr more_leftovers) } } where env = cec_tidy ctxt @@ -587,14 +589,6 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics DE_Multiplicity mult_co loc -> (es1, es2, es3, (mult_co, loc):es4) - -- See Note [Suppressing confusing errors] - suppress :: ErrorItem -> Bool - suppress item - | Wanted <- ei_flavour item - = is_ww_fundep_item item - | otherwise - = False - -- report1: ones that should *not* be suppressed by -- an insoluble somewhere else in the tree -- It's crucial that anything that is considered insoluble @@ -656,11 +650,6 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics , ("Dicts", is_dict, False, mkGroupReporter mkDictErr) , ("Quantified", is_qc, False, mkGroupReporter mkQCErr) ] - -- report3: suppressed errors should be reported as categorized by either report1 - -- or report2. Keep this in sync with the suppress function above - report3 = [ ("wanted/wanted fundeps", is_ww_fundep, True, mkGroupReporter mkEqErr) - ] - -- rigid_nom_eq, rigid_nom_tv_eq, is_dict, is_equality, is_ip, is_FRR, is_irred :: ErrorItem -> Pred -> Bool @@ -728,10 +717,6 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics is_qc _ (ForAllPred {}) = True is_qc _ _ = False - -- See situation (1) of Note [Suppressing confusing errors] - is_ww_fundep item _ = is_ww_fundep_item item - is_ww_fundep_item = isWantedWantedFunDepOrigin . errorItemOrigin - given_eq_spec -- See Note [Given errors] | has_gadt_match_here = ("insoluble1a", is_given_eq, True, mkGivenErrorReporter) @@ -757,6 +742,16 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics = has_gadt_match implics --------------- +suppressItem :: ErrorItem -> Bool + -- See Note [Suppressing confusing errors] +suppressItem item + | Wanted <- ei_flavour item + , let orig = errorItemOrigin item + = isWantedSuperclassOrigin orig -- See (SCE1) + || isWantedWantedFunDepOrigin orig -- See (SCE2) + | otherwise + = False + isSkolemTy :: TcLevel -> Type -> Bool -- The type is a skolem tyvar isSkolemTy tc_lvl ty @@ -781,7 +776,23 @@ If there are any other errors to report, at all, we want to suppress these. Which errors (only 1 case right now): -1) Errors which arise from the interaction of two Wanted fun-dep constraints. +(SCE1) Superclasses of Wanteds. These are generated on in case they trigger functional + dependencies. If such a constraint is unsolved, then its "parent" constraint must + also be unsolved, and is much more informative to the user. Example (#26255): + class (MinVersion <= F era) => Era era where { ... } + f :: forall era. EraFamily era -> IO () + f = ..blah... -- [W] Era era + Here we have simply omitted "Era era =>" from f's type. But we'll end up with + /two/ Wanted constraints: + [W] d1 : Era era + [W] d2 : MinVersion <= F era -- Superclass of d1 + We definitely want to report d1 and not d2! Happily it's easy to filter out those + superclass-Wanteds, becuase their Origin betrays them. + + See test T18851 for an example of how it is (just, barely) possible for the /only/ + errors to be superclass-of-Wanted constraints. + +(SCE2) Errors which arise from the interaction of two Wanted fun-dep constraints. Example: class C a b | a -> b where @@ -852,18 +863,8 @@ Currently, the constraints to ignore are: If there is any trouble, checkValidFamInst bleats, aborting compilation. -(CIG2) Superclasses of Wanteds. These are generated on in case they trigger functional - dependencies. If such a constraint is unsolved, then its "parent" constraint must - also be unsolved, and is much more informative to the user. Example (#26255): - class (MinVersion <= F era) => Era era where { ... } - f :: forall era. EraFamily era -> IO () - f = ..blah... -- [W] Era era - Here we have simply omitted "Era era =>" from f's type. But we'll end up with - /two/ Wanted constraints: - [W] d1 : Era era - [W] d2 : MinVersion <= F era -- Superclass of d1 - We definitely want to report d1 and not d2! Happily it's easy to filter out those - superclass-Wanteds, becuase their Origin betrays them. +(Note: Aug 25: this seems a rather tricky corner; + c.f. Note [Suppressing confusing errors]) Note [Implementation of Unsatisfiable constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ===================================== testsuite/tests/typecheck/should_fail/T18851.hs ===================================== @@ -33,3 +33,14 @@ f = show (undefined :: c) -- blows up at run time once type checks g :: String g = f @A @B + +{- +[W] Show c, Num int, C int A, C int B, C int c +Superclasses + C_FD int ~ A + C_FD int ~ B + C_FD int ~ c +--> + C_FD int ~ B + B ~ A +-} ===================================== testsuite/tests/typecheck/should_fail/UnliftedNewtypesFamilyKindFail2.stderr ===================================== @@ -1,3 +1,7 @@ +UnliftedNewtypesFamilyKindFail2.hs:12:1: error: [GHC-83865] + • Expected a type, but ‘F 5’ has kind ‘5’ + • In the newtype family instance declaration for ‘F’ + UnliftedNewtypesFamilyKindFail2.hs:12:20: error: [GHC-83865] • Expected a type, but ‘5’ has kind View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/745cae95484f32660086d8abeb88bf4a... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/745cae95484f32660086d8abeb88bf4a... 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