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| author | Richard Eisenberg <rae@richarde.dev> | 2021-11-22 17:34:32 -0500 |
|---|---|---|
| committer | Simon Peyton Jones <simon.peytonjones@gmail.com> | 2023-01-11 08:30:42 +0000 |
| commit | aed1974e92366ab8e117734f308505684f70cddf (patch) | |
| tree | bbfe7fdd00f1e0ef8dacdcf8d070a07efa38561b /compiler/GHC/Tc/Solver/InertSet.hs | |
| parent | 083f701553852c4460159cd6deb2515d3373714d (diff) | |
| download | haskell-wip/T20666.tar.gz | |
Refactor the treatment of loopy superclass dictswip/T20666
This patch completely re-engineers how we deal with loopy superclass
dictionaries in instance declarations. It fixes #20666 and #19690
The highlights are
* Recognise that the loopy-superclass business should use precisely
the Paterson conditions. This is much much nicer. See
Note [Recursive superclasses] in GHC.Tc.TyCl.Instance
* With that in mind, define "Paterson-smaller" in
Note [Paterson conditions] in GHC.Tc.Validity, and the new
data type `PatersonSize` in GHC.Tc.Utils.TcType, along with
functions to compute and compare PatsonSizes
* Use the new PatersonSize stuff when solving superclass constraints
See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
* In GHC.Tc.Solver.Monad.lookupInInerts, add a missing call to
prohibitedSuperClassSolve. This was the original cause of #20666.
* Treat (TypeError "stuff") as having PatersonSize zero. See
Note [Paterson size for type family applications] in GHC.Tc.Utils.TcType.
* Treat the head of a Wanted quantified constraint in the same way
as the superclass of an instance decl; this is what fixes #19690.
See GHC.Tc.Solver.Canonical Note [Solving a Wanted forall-constraint]
(Thanks to Matthew Craven for this insight.)
This entailed refactoring the GivenSc constructor of CtOrigin a bit,
to say whether it comes from an instance decl or quantified constraint.
* Some refactoring way in which redundant constraints are reported; we
don't want to complain about the extra, apparently-redundant
constraints that we must add to an instance decl because of the
loopy-superclass thing. I moved some work from GHC.Tc.Errors to
GHC.Tc.Solver.
* Add a new section to the user manual to describe the loopy
superclass issue and what rules it follows.
Diffstat (limited to 'compiler/GHC/Tc/Solver/InertSet.hs')
| -rw-r--r-- | compiler/GHC/Tc/Solver/InertSet.hs | 17 |
1 files changed, 11 insertions, 6 deletions
diff --git a/compiler/GHC/Tc/Solver/InertSet.hs b/compiler/GHC/Tc/Solver/InertSet.hs index b3dcb3f5b1..5dc3431b9a 100644 --- a/compiler/GHC/Tc/Solver/InertSet.hs +++ b/compiler/GHC/Tc/Solver/InertSet.hs @@ -1633,12 +1633,17 @@ mightEqualLater inert_set given_pred given_loc wanted_pred wanted_loc = False can_unify lhs_tv _other _rhs_ty = mentions_meta_ty_var lhs_tv -prohibitedSuperClassSolve :: CtLoc -> CtLoc -> Bool --- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance -prohibitedSuperClassSolve from_loc solve_loc - | InstSCOrigin _ given_size <- ctLocOrigin from_loc - , ScOrigin wanted_size <- ctLocOrigin solve_loc - = given_size >= wanted_size +prohibitedSuperClassSolve :: CtLoc -- ^ is it loopy to use this one ... + -> CtLoc -- ^ ... to solve this one? + -> Bool -- ^ True ==> don't solve it +-- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance, (sc2) +prohibitedSuperClassSolve given_loc wanted_loc + | GivenSCOrigin _ _ blocked <- ctLocOrigin given_loc + , blocked + , ScOrigin _ NakedSc <- ctLocOrigin wanted_loc + = True -- Prohibited if the Wanted is a superclass + -- and the Given has come via a superclass selection from + -- a predicate bigger than the head | otherwise = False |