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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:

@@ -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

 +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