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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 /testsuite/tests/typecheck/should_fail/T20666a.stderr | |
| parent | 083f701553852c4460159cd6deb2515d3373714d (diff) | |
| download | haskell-aed1974e92366ab8e117734f308505684f70cddf.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 'testsuite/tests/typecheck/should_fail/T20666a.stderr')
| -rw-r--r-- | testsuite/tests/typecheck/should_fail/T20666a.stderr | 10 |
1 files changed, 10 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_fail/T20666a.stderr b/testsuite/tests/typecheck/should_fail/T20666a.stderr new file mode 100644 index 0000000000..4192b88807 --- /dev/null +++ b/testsuite/tests/typecheck/should_fail/T20666a.stderr @@ -0,0 +1,10 @@ + +T20666a.hs:11:10: error: [GHC-39999] + • Could not deduce ‘Eq (F [a])’ + arising from the superclasses of an instance declaration + from the context: D [a] + bound by the instance declaration at T20666a.hs:11:10-23 + Possible fix: + If the constraint looks soluble from a superclass of the instance context, + read 'Undecidable instances and loopy superclasses' in the user manual + • In the instance declaration for ‘C [a]’ |